Method and apparatus for achieving higher frequencies of exactly rounded results

ABSTRACT

A multiplier configured to obtain higher frequencies of exactly rounded results by adding an adjustment constant to intermediate products generated during iterative multiplication operations is disclosed. One such iterative multiplication operation is the Newton-Raphson iteration, which may be utilized by the multiplier to perform reciprocal calculations and reciprocal square root calculations. For each iteration, the results converge toward an infinitely precise result. To improve the frequency of the exactly rounded result, the results of the iterative calculations may be studied for a large number of differing input operands to determine the best suited value for the adjustment constant. The multiplier may also be configured to perform scalar and packed vector multiplication using the same hardware.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of microprocessors and,more particularly, to rounding the results of iterative calculations inmicroprocessors.

2. Description of the Related Art

Microprocessors are typically designed with a number of "executionunits" that are each optimized to perform a particular set of functionsor instructions. For example, one or more execution units within amicroprocessor may be optimized to perform memory accesses, i.e., loadand store operations. Other execution units may be optimized to performgeneral arithmetic and logic functions, e.g., shifts and compares. Manymicroprocessors also have specialized execution units configured toperform more complex arithmetic operations such as multiplication andreciprocal operations. These specialized execution units typicallycomprise hardware that is optimized to perform one or more particulararithmetic functions. In the case of multiplication, the optimizedhardware is typically referred to as a "multiplier."

In older microprocessors, multipliers were implemented using designsthat conserved die space at the expense of arithmetic performance. Untilrecently, this was not a major problem because most applications, i.e.,non-scientific applications such as word processors, did not frequentlygenerate multiplication instructions. However, recent advances incomputer technology and software are placing greater emphasis uponmultiplier performance. For example, three dimensional computergraphics, rendering, and multimedia applications all rely heavily upon amicroprocessor's arithmetic capabilities, particularly multiplicationand multiplication-related operations. As a result, in recent yearsmicroprocessor designers have favored performance-oriented designs thatuse more die space. Unfortunately, the increased die space needed forthese high performance multipliers reduces the space available for otherexecution units within the microprocessor. Thus, a mechanism forincreasing multiplier performance while conserving die space in needed.

The die space used by multipliers is of particular importance tomicroprocessor designers because many microprocessors, e.g., thoseconfigured to execute MMX™ (multimedia extension) or 3D graphicsinstructions, may use more than one multiplier. MMX and 3D graphicsinstructions are often implemented as "vectored" instructions. Vectoredinstructions have operands that are partitioned into separate sections,each of which is independently operated upon. For example, a vectoredmultiply instruction may operate upon a pair of 32-bit operands, each ofwhich is partitioned into two 16-bit sections or four 8-bit sections.Upon execution of a vectored multiply instruction, correspondingsections of each operand are independently multiplied. FIG. 1illustrates the differences between a scalar (i.e., non-vectored)multiplication and a vector multiplication. To quickly execute vectoredmultiply instructions, many microprocessors use a number of multipliersin parallel. In order to conserve die space, a mechanism for reducingthe number of multipliers in a microprocessor is desirable. Furthermore,a mechanism for reducing the amount of support hardware (e.g., buslines) that may be required for each multiplier is also desirable.

Another factor that may affect the number of multipliers used within amicroprocessor is the microprocessor's ability to operate upon multipledata types. Most microprocessors must support multiple data types. Forexample, x86 compatible microprocessors must execute instructions thatare defined to operate upon an integer data type and instructions thatare defined to operate upon floating point data types. Floating pointdata can represent numbers within a much larger range than integer data.For example, a 32-bit signed integer can represent the integers between-2³¹ and 2³¹ -1 (using two's complement format). In contrast, a 32-bit("single precision") floating point number as defined by the Instituteof Electrical and Electronic Engineers (IEEE) Standard 754 has a range(in normalized format) from 2⁻¹²⁶ to 2¹²⁷ ×(2-2⁻²³) in both positive andnegative numbers. While both integer and floating point data types arecapable of representing positive and negative values, integers areconsidered to be "signed" for multiplication purposes, while floatingpoint numbers are considered to be "unsigned." Integers are consideredto be signed because they are stored in two's complement representation.

Turning now to FIG. 2A, an exemplary format for an 8-bit integer 100 isshown. As illustrated in the figure, negative integers are representedusing the two's complement format 104. To negate an integer, all bitsare inverted to obtain the one's complement format 102. A constant ofone is then added to the least significant bit (LSB).

Turning now to FIG. 2B, an exemplary format for a 32-bit (singleprecision) floating point number is shown. A floating point number isrepresented by a significand, an exponent and a sign bit. The base forthe floating point number is raised to the power of the exponent andmultiplied by the significand to arrive at the number represented. Inmicroprocessors, base 2 is typically used. The significand comprises anumber of bits used to represent the most significant digits of thenumber. Typically, the significand comprises one bit to the left of theradix point and the remaining bits to the right of the radix point. Inorder to save space, the bit to the left of the radix point, known asthe integer bit, is not explicitly stored. Instead, it is implied in theformat of the number. Additional information regarding floating pointnumbers and operations performed thereon may be obtained in IEEEStandard 754. Unlike the integer representation, two's complement formatis not typically used in the floating point representation. Instead,sign and magnitude form are used. Thus, only the sign bit is changedwhen converting from a positive value 106 to a negative value 108. Forthis reason, many microprocessors use two multipliers, i.e., one forsigned values (two's complement format) and another for unsigned values(sign and magnitude format). Thus, a mechanism for increasing floatingpoint, integer, and vector multiplier performance while conserving diespace is needed.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by a multiplierconfigured in accordance with the present invention. In one embodiment,the multiplier may perform signed and unsigned scalar and vectormultiplication using the same hardware. The multiplier may receiveeither signed or unsigned operands in either scalar or packed vectorformat and accordingly output a signed or unsigned result that is eithera scalar or a vector quantity. Advantageously, this embodiment mayreduce the total number of multipliers needed within a microprocessorbecause it may be shared by execution units and perform both scalar andvector multiplication. This space savings may in turn allow designers tooptimize the multiplier for speed without fear of using too much diespace.

In another embodiment, speed may be increased by configuring themultiplier to perform fast rounding and normalization. This may beaccomplished configuring the multiplier to calculate two version of anoperand, e.g., an overflow version and a non-overflow version, inparallel.

In other embodiments, the multiplier may be further optimized to performcertain calculations such as evaluating constant powers of an operand(e.g., reciprocal or reciprocal square root operations). Iterativeformulas may be used to recast these operations into multiplicationoperations. Iterative formulas generate intermediate products which areused in subsequent iterations to achieve greater accuracy. In someembodiments, the multiplier may be configured to store theseintermediate products for future iterations. Advantageously, someembodiments of the multiplier may be configured to compress theseintermediate products before storing them, which may further conservedie space.

In one embodiment, the multiplier may comprise a partial productgenerator, a selection logic unit, and an adder. The multiplier may alsocomprise a multiplicand input configured to receive a multiplicandoperand (signed or unsigned), a multiplier input configured to receive amultiplier operand (also signed or unsigned), and a sign-in input. Thesign-in input is configured to receive a sign-in signal indicative ofwhether the multiplier is to perform signed or unsigned multiplication.The partial product generator, which is coupled to the multiplicandinput, is configured to generate a plurality of partial products basedupon the multiplicand operand. The selection logic unit, which iscoupled to the partial product generator and the multiplier input, isconfigured to select a number of partial products from the partialproduct generator based upon the multiplier operand. The adder, which iscoupled to the selection logic unit, is configured to sum the selectedpartial products to form a final product. The final product, which maybe signed or unsigned, may then be output to other parts of themicroprocessor.

In addition, the multiplier may further comprise an "effective sign"calculation unit. In one embodiment, the calculation unit may comprise apair of AND gates, each configured to receive the most significant bitof one operand and the sign-in signal. The output of each AND gate isused as the effective sign for that gate's operand. The effective signmay be appended to each operand for use as the operand's sign during themultiplication process. Advantageously, the effective sign may allowboth unsigned operands and signed operands to be multiplied on the samehardware.

A method for operating a multiplier within a microprocessor is alsocontemplated. In one embodiment, the method comprises receiving amultiplier operand, a multiplicand operand, and a sign-in signal fromother functional units within the microprocessor. An effective sign bitfor the multiplicand operand is generated from the sign-in signal andthe most significant bit of the multiplicand operand. A plurality ofpartial products may then be calculated from the effective sign bit andthe multiplicand operand. Next, a number of the partial products may beselected according to the multiplier operand. The partial products arethen summed, and the results are output. In other embodiments, the stepsmay be performed in parallel or in a different order.

In another embodiment, the multiplier may be capable of multiplying onepair of N-bit operands or two pairs of N/2-bit operands simultaneously.The multiplier may comprise a multiplier input and a multiplicand input,each configured to receive an operand comprising one N-bit value or twoN/2-bit values. The multiplier may also comprise a partial productgenerator coupled to the multiplicand input, wherein the partial productgenerator is configured to generate a plurality of partial productsbased upon the value of the multiplicand operand. The multiplier mayfurther comprise a selection logic unit coupled to the partial productgenerator and the multiplier input. The selection logic unit may beconfigured to select a plurality of partial products from the partialproduct generator based upon the value of the multiplier operand. Anadder may be coupled to the selection logic unit to receive and sum theselected partial products to form a final product comprising either one2N-bit value or two N-bit values. The multiplier may receive a vector₋₋in signal indicating whether vector or scalar multiplication is to beformed.

A method for operating a multiplier capable of scalar and vectormultiplication is also contemplated. The method may comprise receiving amultiplier operand, a multiplicand operand, and a vector-in signal asinputs from functional units within the microprocessor and thencalculating a number of partial products from the multiplicand operandusing inverters and shifting logic. Certain partial products may beselected according to the multiplier operand. The selected partialproducts may then be summed to generate a final product. The finalproduct may be in scalar form if the vector₋₋ in signal is unasserted,and in vector form if the vector₋₋ in signal is asserted.

In another embodiment, the multiplier may also be configured tocalculate vector dot products and may comprise a multiplier input and amultiplicand input, each configured to receive a vector. A partialproduct generator may be coupled to the multiplicand input and may beconfigured to generate a plurality of partial products based upon one ofthe vectors. A first adder may be coupled to receive the partialproducts and sum them to generate vector component products for eachpair of vector components. A second adder may be coupled to the firstadder and may be configured to receive and sum the vector componentproducts to form a sum value and a carry value. A third adder may beconfigured to receive the sum and carry values and one or more vectorcomponent products from the first adder. The third adder may beconfigured to output the sum of the sum and carry values (and any carrybits resulting from the summation of the one or more vector components)as a final result.

In yet another embodiment, the multiplier may be configured to outputthe results in segments or portions. This may advantageously reduce theamount of interface logic and the number of bus lines needed to supportthe multiplier. Furthermore, the segments or portions may be rounded. Inthis embodiment, the multiplier may comprise a multiplier input, amultiplicand input, and a partial product generator. The generator iscoupled to the multiplicand input and is configured to generate one ormore partial products. An adder, coupled to the partial productgenerator and the multiplier input, may be configured to receive anumber of the partial products. The adder may sum the partial productstogether with rounding constants to form a plurality of vector componentproducts which are logically divided into portions. One or more of theportions may be rounded.

In another embodiment, the multiplier may be configured to round itsoutputs in a number of different modes. Thus, an apparatus and methodfor rounding and normalizing results within a multiplier is alsocontemplated. In one embodiment, the apparatus comprises an adderconfigured to receive a plurality of redundant-form components. Theadder is configured to sum the redundant-form components to generate afirst non-redundant-form result. The adder may also be configured togenerate a second non-redundant-form result comprising the sum of theredundant-form components plus a constant. Two shifters are configuredto receive the results. Both shifters may be controlled by the mostsignificant bits of the results they receive. A multiplexer may becoupled to receive the output from the shifters and select one of themfor output based upon the least significant bits in the firstnon-redundant-form result. By generating more than version of the result(e.g., the result and the result plus a constant) in parallel, roundingmay be accomplished in less time than previously required.

A multiplier configured to round and normalize products is alsocontemplated. In one embodiment, the multiplier may comprise two paths.Each path may comprise one or more adders, each configured to receive aredundant-form product and reduce it to a non-redundant form. The firstpath does so assuming no overflow will occur, while the second path doesso assuming an overflow will occur. A multiplexer may be coupled to theoutputs of the two paths, so as to select between the results from thefirst and second paths.

A method for rounding and normalizing results within a multiplier isalso contemplated. In one embodiment, the method comprises multiplying afirst operand and a second operand to form a plurality of redundant-formcomponents. A rounding constant is generated and added to theredundant-form component in two different bit positions. The firstposition assumes an overflow will occur, while the second positionassumes no overflow will occur. A particular set of bits are selectedfor output as the final result from either the first addition or thesecond addition.

Also contemplated is an apparatus for evaluating a constant power of anoperand using a multiplier. In one embodiment, the apparatus comprisesan initial estimate generator configured to receive the operand andoutput an initial estimate of the operand raised to the desired constantpower. A multiplier may be coupled to receive the operand and theinitial estimate, wherein the multiplier is configured to calculate theproduct of the initial estimate and the operand. A first plurality ofinverters may be coupled to receive, invert, and normalize selected bitsfrom the product to form a first approximation, wherein the firstapproximation assumes an overflow has occurred in the multiplier. Asecond plurality of inverters may be coupled to receive, invert, andnormalize selected bits from the product to form a second approximation,wherein the second approximation assumes an overflow has not occurred inthe multiplier. A multiplexer may be configured to select either thefirst or second approximations for output.

Also contemplated is a method for evaluating a constant power of anoperand using a multiplier. In one embodiment, the method comprisesdetermining an initial estimate of the operand raised to a firstconstant power. The operand and the initial estimate are then multipliedin the multiplier to form a first product. A normalized firstintermediate approximation is calculated by performing a bit-wiseinversion on the first product assuming an overflow occurred during themultiplying. A normalized second intermediate approximation iscalculated by performing a bit-wise inversion on the first productassuming no overflow occurred during the multiplying. Finally, a set ofbits are selected from either the first intermediate approximation orthe second intermediate approximation to form a selected approximationthat may be output or used in subsequent iterations to achieve a moreaccurate result.

An apparatus for rounding and normalizing a redundant-form value is alsocontemplated. In one embodiment, the apparatus may comprise two addersand a multiplexer. The first adder is configured to receive theredundant-form value and add a rounding constant to its guard bitposition, thereby forming a first rounded result, wherein the guard bitposition is selected assuming no overflow will occur. The second adderis similarly configured and performs the same addition assuming,however, that an overflow will occur. A multiplexer is configured toselect either the first rounded result or the second rounded resultbased upon one or more of the most significant bits from the first andsecond rounded results. Performing the rounding in parallel mayadvantageously speed the process by allowing normalization to take placein parallel with the multiplexer's selection.

A method for operating a multiplier that compresses intermediate resultsis also contemplated. In one embodiment, this method comprisescalculating an intermediate product to a predetermined number of bits ofaccuracy. Next, a signaling bit is selected from the intermediateproduct. The signaling bit is equal to each of the N most significantbits of the intermediate product. Next, the intermediate product iscompressed by replacing the N most significant bits of the intermediateproduct with the signaling bit. The compressed intermediate product isthen stored into a storage register. During the next iteration, thestorage register is read to determine the value of the compressedintermediate product. The compressed intermediate product may beexpanded to form an expanded intermediate product by padding thecompressed intermediate product with copies of the signaling bit.

A multiplier configured to perform iterative calculations and tocompress intermediate products is also contemplated. In one embodiment,the multiplier comprises a multiplier input, a multiplicand input, and apartial product generator as described in previous embodiments. Themultiplier also comprises a partial product array adder which isconfigured to receive and add a selected plurality of partial productsto form an intermediate product. Compression logic may be coupled to thepartial product array adder. The compression logic may comprise a wireshifter configured to replace a predetermined number of bits of theintermediate product with a single signal bit, which represents theinformation stored in the predetermined number of bits. The signal bitis selected so that it equals the value of each individual bit withinthe predetermined number of bits. A storage register may be coupled toreceive and store the compressed intermediate product from thecompression logic.

In another embodiment, the multiplier may be configured to add anadjustment constant to increase the frequency of exactly roundedresults. In such an embodiment, the multiplier may comprise a multiplierinput configured to receive a multiplier operand, a multiplicand inputconfigured to receive a multiplicand operand, a partial productgenerator, and selection logic. In one embodiment, the partial productgenerator is coupied to the multiplicand input and configured togenerate one or more partial products based upon the multiplicandoperand. The selection logic may be coupled to the partial productgenerator and the multiplier, wherein the selection logic is configuredto select a plurality of partial products based upon the multiplier. Thepartial product array adder may be coupled to the selection logic,wherein the adder is configured to receive and sum a number of thepartial products and an adjustment constant to form a product. Theadjustment constant is selected to increase the frequency that theresult is exactly rounded.

A method for increasing the frequency of exactly rounded results is alsocontemplated. In one embodiment, the method comprises receiving anoperand and determining an initial estimate of the result of aniterative calculation using the operand. The initial estimate and theoperand are multiplied to generate an intermediate result. Themultiplication is repeated a predetermined number of times, wherein theintermediate result is used in place of the initial estimate insubsequent iterations. The final repetition generates a final result,and an adjustment constant may be added to the final result, wherein theadjustment constant increases the probability that the final result willequal the exactly rounded result of the iterative calculation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a diagram illustrating an exemplary scalar multiplication andan exemplary vector multiplication.

FIG. 2A is a diagram of an exemplary integer data format using two'scomplement representation.

FIG. 2B is a diagram of an exemplary floating point data format.

FIG. 3 is a block diagram of one embodiment of an exemplarymicroprocessor.

FIG. 4 is a block diagram of one embodiment of the computational corefrom the microprocessor of FIG. 3.

FIG. 5A illustrates one embodiment of the shift-and-add algorithm forbinary multiplication.

FIG. 5B illustrates one embodiment of Booth's algorithm for binarymultiplication.

FIG. 6 is a block diagram illustrating details of one embodiment of themultiplier from FIG. 4.

FIG. 7 is a block diagram illustrating the operation of the multiplierfrom FIG. 6 for unsigned operands.

FIG. 8 is a block diagram illustrating an example of the operation ofthe multiplier from FIG. 6 for signed operands.

FIG. 9 is a block diagram illustrating another example of the operationof the multiplier from FIG. 6 for signed operands.

FIG. 10 is a diagram illustrating one embodiment of the multiplier fromFIG. 4 that is configured to perform vector multiplication.

FIG. 11A is a diagram that illustrates details of one embodiment of thepartial product generator from FIG. 6.

FIG. 11B is a diagram that illustrates in detail part of one embodimentof the selection logic from FIG. 6.

FIG. 12A-B is a diagram that illustrates details of one embodiment ofthe selection logic and adder from FIG. 6.

FIG. 13 is a diagram illustrating another embodiment of the multiplierfrom FIG. 4 that is configured to perform vector multiplication.

FIG. 14 is a diagram illustrating yet another embodiment of themultiplier from FIG. 4 that is configured to perform vectormultiplication.

FIG. 15 is a diagram illustrating one embodiment of a multiplier that isconfigured to calculate the vector dot product of a pair of vectoroperands.

FIG. 16 is a diagram illustrating another embodiment of a multiplierthat is configured to calculate the vector dot product of a pair ofvector operands.

FIG. 17 is a diagram illustrating one embodiment of a multiplier that isconfigured to return vector component products in portions, some ofwhich may be rounded.

FIG. 18 is a diagram illustrating another embodiment of a multiplierthat is configured to return vector component products in portions, someof which may be rounded.

FIG. 19 is a diagram illustrating one embodiment of the multiplier fromFIG. 6 configured to perform rounding.

FIG. 20 is a diagram illustrating a numerical example of the operationof the multiplier from FIG. 19.

FIG. 21 is a diagram illustrating details of one embodiment of thesticky bit logic from FIG. 19.

FIG. 22 is a diagram illustrating a numerical example of the operationof the multiplier from FIG. 19.

FIG. 23 is a diagram illustrating another embodiment of the multiplierfrom FIG. 6.

FIG. 24 is a flowchart illustrating one embodiment of a method forcalculating the reciprocal of an operand.

FIG. 25 is a flowchart illustrating one embodiment of a method forcalculating the reciprocal square root of an operand.

FIG. 26 is a diagram illustrating one embodiment of the multiplier fromFIG. 6 that is configured to perform iterative calculations.

FIG. 27 is a diagram illustrating details of one exemplary embodiment ofthe non-overflow and overflow logic units from FIG. 26.

FIG. 28 is a diagram illustrating details of another exemplaryembodiment of non-overflow and overflow logic units from FIG. 26.

FIG. 29A is a flowchart illustrating one possible method for fastcompression.

FIG. 29B is a flowchart illustrating one possible method for fastdecompression.

FIG. 30 is a diagram illustrating one embodiment of the multiplier fromFIG. 4 configured to compress intermediate products.

FIG. 31A is a figure illustrating one possible method for compression.

FIG. 31B is a figure illustrating another possible method forcompression.

FIG. 32 is a figure illustrating one embodiment of a multiplierconfigured to achieve a higher frequency of exactly rounded results.

FIG. 33A is a diagram illustrating an example of a vector multiplicationusing two multipliers.

FIG. 33B is a diagram illustrating another example of a multiplicationusing two multipliers.

FIG. 34 is a block diagram of one embodiment of a computer systemconfigured to utilize the microprocessor of FIG. 3.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF AN EMBODIMENT

Turning now to FIG. 3, a block diagram of one embodiment of amicroprocessor 10 is shown. As depicted, microprocessor 10 comprises apredecode logic block 12, a bus interface unit 24, and a level one-cachecontroller 18, all of which are coupled to the following three caches: alevel-one instruction cache 14, a level-one data cache 26, and anon-chip level-two cache 40. Both instruction cache 14 and data cache 26are configured with translation lookaside buffers, i.e., TLBs 16 and 28,respectively. Microprocessor 10 further comprises a decode unit 20 whichreceives instructions from instruction cache 14, decodes them, and thenforwards them to an execution engine 30 in accordance with inputsreceived from a branch logic unit 22.

Execution engine 30 comprises a scheduler buffer 32, an instructioncontrol unit 34, and a plurality of execution units 36A-36F. Note thatblocks referred to herein with a reference number followed by a lettermay be collectively referred to by the reference number alone. Forexample, execution units 36A-F may be collectively referred to asexecution units 36. Scheduler buffer 32 is coupled to receive decodedinstructions from decode unit 20 and convey them to execution units 36in accordance with input received from instruction control unit 34. Inone embodiment, execution units 36A-F include a load unit 36A, a storeunit 36B, two integer/MMX/3D units 36C and 36D, a floating point unit36E, and a branch resolving unit 36F. Load unit 36A receives input fromdata cache 26, while store unit 36B interfaces with data cache 26 via astore queue 38. Integer/MMX/3D units 36C and 36D, and floating pointunit 36E collectively form a computational core 42 for microprocessor10. Computational core 42 may further comprise other execution units andspecialized hardware such as multipliers.

Before describing computational core 42 in detail, other features ofmicroprocessor 10 will be discussed. In one embodiment, instructioncache 14 is organized into sectors, with each sector including two32-byte cache lines. The two cache lines within each sector share acommon tag but have separate state bits that indicate the status of theline. Accordingly, two forms of cache misses (and associated cachefills) may take place: (1) sector replacement and (2) cache linereplacement. In the case of sector replacement, the cache miss is causedby a tag mismatch in instruction cache 14. Thus the required cache lineis supplied by external memory via bus interface unit 24. The cache linewithin the sector that is not needed is then marked invalid. In the caseof a cache line replacement, a tag matches the requested address but thecorresponding cache line is marked as invalid. The required cache lineis then supplied by external memory, but unlike a sector replacement,the cache line within the sector that was not requested remainsunaltered. In alternate embodiments, other organizations and replacementpolicies for instruction cache 14 may be utilized.

In one embodiment, microprocessor 10 may be configured to performprefetching only in the case of sector replacements. During sectorreplacement, the required cache line is filled. If the required cacheline is in the first half of the sector, the other cache line in thesector is prefetched. If the required cache line is in the second halfof the sector, no prefetching is performed. Other prefetchingmethodologies may also be employed in different embodiments ofmicroprocessor 10.

When cache lines of instruction data are retrieved from external memoryby bus interface unit 24, the data is conveyed to predecode logic block12. In one embodiment, the instructions processed by microprocessor 10and stored in cache 14 are variable-length (e.g., the x86 instructionset). Because decoding variable-length instructions is particularlycomplex, predecode logic 12 may be configured to provide additionalinformation to be stored in instruction cache 14 to aid during decode.In one embodiment, predecode logic 12 generates "predecode bits" foreach byte in instruction cache 14. The predecode bits may providevarious information useful during the decode process, e.g., the numberof bytes to the start of the next variable-length instruction. Thepredecode bits are passed to decode unit 20 when instruction bytes arerequested from cache 14.

In one embodiment, instruction cache 14 is implemented as a 32-Kbyte,two-way set-associative, writeback cache. The cache line size may be 32bytes in this embodiment. Cache 14 also includes a 64-entry TLB that maybe used to speed linear to physical address translation. Othervariations of instruction cache 14 are possible and contemplated.

Instruction cache 14 receives instruction fetch addresses from cachecontroller 18. In one embodiment, up to 16 bytes may be fetched fromcache 14 per clock cycle. The fetched information is placed into aninstruction buffer that feeds into decode unit 20. In one embodiment ofmicroprocessor 10, fetching may occur along a single execution streamwith seven outstanding branches taken. In another embodiment, fetchingmay take place along multiple execution streams.

In one embodiment, the instruction fetch logic within cache controller18 is capable of retrieving any 16 contiguous instruction bytes within a32-byte boundary of cache 14 with no additional penalty when the 16bytes cross a cache line boundary. New instructions are loaded into theinstruction buffer as the current instructions are consumed by decodeunit 20. Other configurations of cache controller 18 are also possibleand contemplated.

In one embodiment, decode logic 20 may be configured to decode multipleinstructions per processor clock cycle. Decode unit 20 may further beconfigured to accept instruction and predecode bytes from theinstruction buffer (in x86 format), locate actual instructionboundaries, and generates corresponding "RISC ops". RISC ops arefixed-format internal instructions, most of which are executable bymicroprocessor 10 in a single clock cycle. In one embodiment ofmicroprocessor 10, RISC ops are combined to form every function in thex86 instruction set. Microprocessor 10 may use a combination of decodersto convert x86 instructions into RISC ops. In one embodiment, thehardware comprises three sets of decoders: two parallel short decoders,one long decoder, and one vector decoder. The parallel short decoderstranslate the most commonly-used x86 instructions (e.g., moves, shifts,branches, etc.) into zero, one, or two RISC ops each. The short decodersonly operate on x86 instructions that are up to seven bytes long. Inaddition, they are configured to decode up to two x86 instructions perclock cycle. Commonly-used x86 instructions which are greater than sevenbytes long, as well as those semi-commonly-used instructions that are upto seven bytes long, are handled by the long decoder.

The long decoder in decode unit 20 only performs one decode per clockcycle generating up to four RISC ops. All other translations (complexinstructions, interrupts, etc.) are handled by a combination of thevector decoder and an on-chip ROM. For complex operations, the vectordecoder logic provides the first set of RISC ops and an initial addressto a sequence of further RISC ops within the on-chip ROM. The RISC opsfetched from the on-chip ROM are of the same type that are generated bythe hardware decoders.

In one embodiment, decode unit 20 generates a group of four RISC opseach clock cycle. For clock cycles in which four RISC ops cannot begenerated, decode unit 20 places RISC NOP operations in the remainingslots of the grouping. These groupings of RISC ops (and possible NOPs)are then conveyed to scheduler buffer 32. It is noted that in otherembodiments, microprocessor 10 may be configured to decode otherinstructions sets in place of, or in addition to, the x86 instructionset.

Instruction control logic 34 contains the logic necessary to manageout-of-order execution of instructions stored in scheduler buffer 32.Instruction control logic 34 also manages data forwarding, registerrenaming, simultaneous issue and retirement of RISC ops, and speculativeexecution. In one embodiment, scheduler buffer 32 holds up to 24 RISCops at one time, which is equivalent to a maximum of twelve x86instructions. When possible, instruction control logic 34 maysimultaneously issue (from buffer 32) RISC ops to any availableexecution units 36. In one embodiment, control logic 34 may beconfigured to issue up to six and retire up to four RISC ops per clockcycle.

In one embodiment, store unit 36A and load unit 36B may each havetwo-stage pipelines. Store unit 36A may be configured to perform memoryand register writes such that the data is available for loading afterone clock cycle. Similarly, load unit 36B may be configured to performmemory reads such that the data is available after two clock cycles.Other configurations for load and store units 36A and 36B are alsopossible with varying latencies.

Execution unit 36G (the branch resolving unit) is separate from branchprediction logic 22 in that it resolves conditional branches such as JCCand LOOP after the branch condition has been evaluated. Branch resolvingunit 36G allows efficient speculative execution, enabling microprocessor10 to execute instructions beyond conditional branches before knowingwhether the branch prediction was correct. As described above,microprocessor 10 may be configured to handle up to seven outstandingbranches in one embodiment.

Branch prediction logic 22, coupled to decode unit 20, is configured toincrease the accuracy with which conditional branches are predicted inmicroprocessor 10. Ten to twenty percent of the instructions in typicalapplications include conditional branches. Branch prediction logic 22 isconfigured to handle this type of program behavior and its negativeeffects on instruction execution, such as stalls due to delayedinstruction fetching. In one embodiment, branch prediction logic 22includes an 8192-entry branch history table, a 16-entry by 16 bytebranch target cache, and a 16-entry return address stack. Branchprediction logic 22 may implement a two-level adaptive history algorithmusing the branch history table. The table stores executed branchinformation, predicts individual branches, and predicts behavior ofgroups of branches. In one embodiment, the branch history table does notstore predicted target addresses in order to save space. Instead, theaddresses are calculated on-the-fly during the decode stage.

To avoid a clock cycle penalty for a cache fetch when a branch ispredicted taken, a branch target cache within branch logic 22 suppliesthe first 16 bytes at that address directly to the instruction buffer(assuming a hit occurs in the branch target cache). In one embodiment,branch prediction logic 22 achieves branch prediction rates of over 95%.

Branch logic 22 may also include special circuitry designed to optimizethe CALL and RET instructions. This circuitry allows the address of thenext instruction following the CALL instruction in memory to be pushedonto a return address stack. When microprocessor 10 encounters a RETinstruction, branch logic 22 pops this address from the return stack andbegins fetching.

Like instruction cache 14, data cache 26 may also be organized astwo-way set associative 32-Kbyte storage. In one embodiment, data TLB 28includes 128 entries that may be used to translate linear to physicaladdresses. Like instruction cache 14, data cache 26 may also besectored. Data cache 26 may further implement a MESI(modified-exclusive-shared-invalid) protocol to track cache line status.Other configurations of data cache 26 are also possible and arecontemplated.

Computational Core

Turning now to FIG. 4, more detail of one embodiment of computation core42 is shown. In one embodiment, computation core 42 comprises threeexecution units 36C-E and a multiplier 50. Integer/MMX/3D execution unit36C is a fixed point execution unit which is configured to operate onall ALU operations, as well as multiplies, divides (both signed andunsigned), shifts, and rotates. In contrast, integer/MMX/3D executionunit 36E (Integer Y unit) is a fixed point execution unit configured tooperate only on the basic word and doubleword ALU operations (ADD, AND,CMP, etc.).

Execution units 36C and 36D are also configured to accelerateperformance of software written using multimedia and 3D graphicsinstructions. Applications that can take advantage of multimedia and 3Dgraphics instructions include 3D modeling and rendering, video and audiocompression/decompression, speech recognition, and telephony. Executionunits 36C and 36D may be configured to execute multimedia instructionsin a single clock cycle. Many of these instructions are designed toperform the same operation to multiple sets of data at once (i.e.,vector processing). In one embodiment, execution units 36C and 36D useregisters which are mapped onto the stack of floating point unit 36E.

Execution unit 36E contains an IEEE 754-compatible floating point unitdesigned to accelerate the performance of software which utilizes thex86 instruction set. Floating point software is typically written tomanipulate numbers that are either very large or small, require a greatdeal of precision, or result from complex mathematical operations suchas transcendentals. Floating point execution unit 36E may comprise anadder unit, a multiply unit, and a divide/square root unit. In oneembodiment, these low-latency units are configured to execute floatingpoint instructions in as few as two clock cycles.

In one embodiment, execution units 36C and 36D are coupled to multiplier50 and are configured to utilize multiplier 50 as a shared resource.Advantageously, this configuration allows both execution units 36C and36D to perform multiplication without requiring two multipliers. Inanother configuration, each execution unit 36C and 36D may each havetheir own dedicated multiplier. Still other configurations are possibleand contemplated. For example, two n-bit multipliers may be shared byexecution units 36C and 36D. Configuring microprocessor 10 with twomultipliers each having a width of 32-bits advantageously allows twosingle precision multiplications to be executed in parallel (eachoperand/significand is 24 bits wide), or one MMX packed multiply (i.e.,multiplying a pair of vectors wherein each vector comprises four 16-bitcomponents). In another embodiment, multiplier 50 may be configured toaccept operands that are 76-bits wide (i.e., the width of thesignificand in a double precision floating point data type), therebyproviding the same functionality as two separate 32-bit multiplierswhile further alleviating the need for a separate multiplier in floatingpoint unit 36E. In such an embodiment, execution units 36C-36E may bedirectly coupled to multiplier 50, with each execution unit sharingmultiplier 50.

Multiplier 50 may also be configured to perform both signed and unsignedmultiplication. Advantageously, this allows multiplier 50 to supportboth integer multiplication for MMX instructions, and floating pointmultiplication for 3D graphics instructions.

While multiplier 50 may be configured to perform multiplication using anumber of different algorithms, the embodiment shown in the figure isconfigured to use a modified version of Booth's Algorithm to improvemultiplication times. Booth's algorithm relies upon calculating a numberof partial products and then summing them to obtain a final product.Booth's algorithm is able to improve multiplication times over thestandard "add-and-shift" algorithm by reducing the number of partialproducts that need to be summed in order to obtain the final product.For example, in performing an 8-bit by 8-bit multiplication, theshift-and-add algorithm generates eight partial products. By contrast,same 8-bit by 8-bit multiplication using the 2-bit version of Booth'salgorithm generates only five partial products. This reduction in thenumber of partial products is illustrated in FIGS. 5A and 5B.

Turning now to FIG. 6, more detail of one embodiment of multiplier 50 isshown. In this embodiment, multiplier 50 comprises a partial productgenerator 60, a partial product selection logic unit 62, and an adder64. As shown in the figure, partial product generator 60 is coupled toselection logic unit 62, which is in turn coupled to adder 64. When oneof execution units 36C-36E receives an instruction invoking themultiplication function, the execution unit conveys two operands tomultiplier 50, i.e., a multiplicand operand 72 and a multiplier operand74. Partial product generator 60 is coupled to receive multiplicandoperand 72, which is used as a starting value for calculating aplurality of partial products 70. For example, if partial productgenerator 60 is configured to use the 2-bit version of Booth'salgorithm, the following partial products would be generated: themultiplicand itself ("+M"), a shifted version of the multiplicand("+2M"), an inverted version of the multiplicand ("-M"), a shifted andinverted version of the multiplicand ("-2M"), and two constants, i.e., apositive zero ("+0") and a negative zero ("-0") in two's complementform.

Partial product selection unit 62 is coupled to receive multiplieroperand 74. Selection unit 62 is configured to select a number ofpartial products from generator 60 based upon particular fields withinmultiplier operand 74. For example, using the 2-bit version of Booth'salgorithm, multiplier operand 74 is padded with leading and trailingzeros (assuming an unsigned multiplication is being performed), and thenone partial product is selected by each 3-bit field within the operand.

Finally, adder 64 is configured to receive and sum the partial productsselected by selection unit 62. As noted in the figure, the selectedpartial products 68 are shifted before they are summed. The resultingfinal product 76 is output to the execution unit that transmitted theoperands. As previously noted, multiplier 50 may advantageously beconfigured to perform both signed and unsigned multiplication. This isdescribed in greater detail below.

Scalar Unsigned Multiplication

Turning now to FIG. 7, details of one embodiment of multiplier 50 areshown. The figure also illustrates the operation of multiplier 50 for anunsigned multiplication. While the figure shows an 8-bit by 8-bitmultiplier using the 2-bit version of Booth's algorithm, otherconfigurations are possible and contemplated, e.g., a 32-bit by 32-bitmultiplier using a 3-bit version of Booth's algorithm. In thisembodiment, multiplier 50 further comprises a "sign-in" input 78, whichindicates whether a signed or unsigned multiplication is to beperformed. Sign-in input 78 is coupled to AND gate 86A, which alsoreceives the most significant bit ("MSB") of multiplier operand 74. ANDgate 86A outputs an "effective sign" bit 90 for multiplier operand 74which is copied and appended to multiplier operand 74 for use byselection logic unit 62. Sign-in input 78 is also routed to AND gate88B, which similarly calculates and appends an effective sign bit 92 formultiplicand operand 72. While other effective sign calculation logicmay be used, the configuration illustrated advantageously generates aneffective sign of zero for all unsigned operands and positive signedoperands using a minimum amount of logic. Furthermore, in the embodimentshown only signed negative operands receive an asserted effective signbit.

Partial product generation logic 60 uses multiplicand operand 72 andeffective sign bit 92 to generate a number of partial products 80A-80C.For example, a shifted version 80A of multiplicand operand 72 isgenerated by shifting logic 84B. Shifted version 80A is equivalent totwo times the multiplicand operand (+2M). Similarly, inverters 98generate an inverted (i.e., one's complement) version (-M) ofmultiplicand operand 72. Shifting logic 84A is used to generate ashifted and inverted version 80C (-2M) of multiplicand operand 72.Partial product generation logic 60 also generates constants for use aspartial products, e.g., positive zero 82B (+0) and negative zero 82A(-0). As illustrated in the figure, each partial product 80A, 80B, 80C,72, 82A, and 82B may have an extra constant bit 88 associated with it.Extra constant bit 88 is asserted only for negative partial products,i.e., -M, -2M, and -0, and is added to the partial product within adder64 to generate two's complement versions of the inverted partialproducts. The shaded areas of the figure denote constants that may bedesigned into multiplier 50.

Once partial product generator 60 has generated the partial products,selection logic 62 is configured to select partial products based upon3-bit fields from multiplier operand 74. Multiplier operand 74 is addedwith zeros and copies of effective sign bit 90 so that there are nofractional 3-bit fields. Selection logic 62 may comprise a number ofmultiplexers 94A-94F, one for each partial product to be selected. Eachmultiplexer 94A-94E is controlled by a different 3-bit field frommultiplier operand 74. The 3-bit fields determine which partial productfrom those generated by partial product generator 60, i.e., +M, +2M, -M,-2M, +0, -0, will be selected. The selected partial products are thenconveyed to adder 64. Using 2-bit Booth decoding, Table 1 describes howpartial products will be selected.

                  TABLE 1                                                         ______________________________________                                        3-bit Multiplier Field Value                                                                   Partial Product Selected                                     ______________________________________                                        000            +0                                                             001            +M                                                             010            +M                                                             011            +2M                                                            100            -2M                                                            101            -M                                                             110            -M                                                             111            -0                                                             ______________________________________                                    

Adder 64 is configured to receive and sum the selected partial products.As illustrated in the figure, the partial products are shifted beforebeing summed. Some of the partial products may have prefix bits added toeliminate the need for sign extending the partial product's mostsignificant bit (i.e., sign bit) to the maximum width of final product76. The prefixes may be generated using simple inverters coupled to thepartial product's most significant bit and constants. Once the partialproducts are shifted, padded, and summed, final product 76 is output andconveyed to the execution unit that provided the operands. Adder 64 mayuse a number of different algorithms for summing the partial products.For example, adder 64 may configured as a carry look-ahead adder, acarry skip adder, a carry select adder, a carry-save adder, or a carrypropagate adder.

The exemplary values in the figure illustrate the unsignedmultiplication of two values, 240₁₀ and 230₁₀. Sign-in input 78 isunasserted because unsigned multiplication to be performed. Sign-ininput 78 may be provided by the same execution unit that provided theoperands. The execution unit may generate sign-in input bit 78 basedupon the type of multiply instruction it received. In the example shownin the figure, effective signs 90 and 92 are both zero because sign-ininput 78 is unasserted. As shown in the illustration, an 8-bit by 8-bitversion of multiplier 50 is able to multiply 8-bit unsigned operands(i.e., operands that do not have a sign bit) having values from 0 to 255to obtain a 16-bit unsigned result.

Scalar Signed Multiplication

Turning now to FIG. 8, the same 8-bit by 8-bit version of multiplier 50is shown. In this figure, however, multiplier 50 is performing signedmultiplication. Sign-in input 78 is asserted because signedmultiplication is to be performed. In the example illustrated,multiplicand operand 72 equals 100₁₀, while multiplier operand 74 equals-50₁₀. Multiplier operand 74 is received in two's complement formatbecause it is a negative signed value. Thus its effective sign bit 90(as calculated by AND gate 88A) is asserted. In contrast, effective signbit 92 for multiplicand operand 72 is unasserted because multiplicandoperand 72 is positive. The final product 76 is a negative 16-bit number(-5000₁₀) represented in two's complement format with the MSB indicatingthe sign.

Turning now to FIG. 9, another example of multiplier 50 performing asigned multiplication is shown. In this example, however, bothmultiplier operand 74 (having a value of -50₁₀) and multiplicand operand72 (having a value of -100₁₀) are received in two's complement format.The multiplication results in a signed final product 76 (having a valueof 5000₁₀) that is positive. As FIGS. 6-8 illustrate, multiplier 50 mayadvantageously perform both signed and unsigned multiplication with thesame hardware. Furthermore, multiplier 50 may advantageously beconfigured to use Booth's algorithm to further increase multiplicationperformance.

Component-Wise Vector Multiplication

As previously noted, recent advances have placed a greater emphasis onmicroprocessors' multimedia and graphics performance. Multimedia and 3Dextensions to the basic x86 instruction set include vectored multiplyinstructions to improve performance. Turning now to FIG. 10, anembodiment of multiplier 50 capable of performing vector multiplicationis shown. As in previous embodiments, multiplier 50 comprises partialproduct generator 60, selection logic 62, and adder 64. This embodimentof multiplier 50 is configured to perform component-wise vectormultiplication of two pairs of N-bit values (A1×B1 and A2×B2)simultaneously or a scalar multiplication of one pair of 2N-bit values(A×B). Advantageously, multiplier 50 may take the place of threeseparate multipliers (i.e., one for scalar multiplication and two forthe vector multiplication), thereby saving valuable die space.

In this embodiment, multiplier 50 has several features which allow it toperform both scalar and component-wise vector multiplication. Whenscalar multiplication is performed, multiplier 50 functions aspreviously disclosed, i.e., adder 64 will sum the partial productsselected by selection logic 62 from partial product generator 60 to formfinal product 76. When performing component-wise vector multiplication,however, multiplier 50 is configured to effectively operate as twoseparate multipliers. This behavior ensures that the results generatedby multiplier 50 will equal the results that would have been generatedhad two separate multipliers been used. To indicate whether multiplier50 should perform component-wise vector multiplication or scalarmultiplication, multiplier 50 receives a vector₋₋ in input signal 120.When an asserted vector₋₋ in signal is received, a plurality ofmultiplexers within selection logic 62 (e.g., multiplexers 122 and 124)effectively isolate the two "logical halves" of multiplier 50. Thisseparation prevents partial products from one pair of vector components(e.g., A1 and B1) from interfering with the multiplication of anotherpair of vector components (e.g., A2 and B2). The operation ofmultiplexers 122 and 124 is described in greater detail below.

As shown in the figure, multiplicand operand 72 and multiplier operand74 may each comprise a vector (two N-bit values) or a scalar value (asingle 2N-bit value). For example, multiplicand operand 72 may comprisea vector (A2, A1) or a single scalar value A. The partial productsselected by selection logic 62 may be logically divided into fourquadrants 130-136 for component-wise vector multiplications (assumingvector operands each having two vector components). Quadrant 130represents the higher order bits of partial products selected by theleast significant vector component of vector multiplier 74 (i.e., B1).Quadrant 132 represents the lower order bits of partial productsselected by the least significant vector component of vector multiplier74 (i.e., B1). Quadrant 134 represents the lower order bits of partialproducts selected by the most significant vector component of vectormultiplier 74 (i.e., B2). Quadrant 136 represents the higher order bitsof partial products selected by the most significant vector component ofvector multiplier 74 (i.e., B2).

As the selected partial products are shifted before being summed inadder 64, the least significant bits of partial products selected byvector component B2 located within quadrant 134 may affect the additionperformed to generate A1×B1 within final product 76. To prevent this"corruption" of final product 76, multiplexer 124 is configured to"zero-out" the lower order bits of partial products located withinquadrant 134. Similarly, in some embodiments the higher order bits ofpartial products selected by vector component B1 may extend intoquadrant 130, thereby possibly affecting the summation used to formB1×B2 within final product 76. Thus additional multiplexers similar tomultiplexer 124 may be used to zero-out the higher order bits withinquadrant 130.

Multiplexer 122 also assists in the logical separation that isadvantageous for component-wise vector multiplication. Staggered bitfields within multiplier operand 74 are used to select partial productsfrom partial product generator 60. When a bit field encompasses bitsfrom more than one vector component within multiplier operand 74, theresulting partial product may also be "corrupted." For example,selecting a partial product using one bit from vector component B1 andtwo bits from vector component B2 (as illustrated in the figure) willresult in a partial product that is partially representative of vectorcomponent B1 and partially representative of vector component B2. Thisis undesirable because B1 is to be multiplied with A1 separately fromB2. To remedy this, a multiplexer 122 may be used. When a bit fieldencompasses bits from more than one vector component, multiplexer 122may zero-out the unwanted bit or bits (e.g., the most significant bitfrom B1 as shown in the figure). Thus, the partial product selected bymultiplexer 94B will reflect only the bit values within the desiredvector component. A second multiplexer similar to multiplexer 122 mayzero out the opposite bits. Thus two partial products may be selected,one representing the end of vector operand B1 and one representing thebeginning of vector operand B2. The zeroing-out of bits for partialproduct selection and summation are illustrated in more detail by way ofa numerical example in FIGS. 11A through 12.

Turning now to FIG. 11A, more detail of one embodiment of partialproduct generator 60 is shown. To support component-wise vectormultiplication when the vector components are signed, an additionaleffective sign bit 172A-172F may be generated for the lower-orderportion of each partial product. The same logic may be used aspreviously disclosed, with AND-gate 86B being duplicated (see AND-gate86C) to generate an effective sign for each lower-order vectorcomponent. Advantageously, multiplier 50 may be configured to performboth signed and unsigned vector multiplication. Generator 60 may also beconfigured to generate separate constant bits 88A-F (referred to as S1)and 170A-F (referred to as S2) to further improve separability when theselected partial products are summed in adder 64. The extra constantbits 170A-F and effective sign bits 172A-F may simply remain unused orunselected during scalar multiplication. Note the figure illustrates onepossible set of partial products generated for an unsignedcomponent-wise vector multiplication wherein the multiplicand operand 72has the values of (6,7), i.e., A2=6 and A1=7. Sign₋₋ in input 78 isunasserted to indicate that an unsigned multiplication is beingperformed.

Turning now to FIG. 11B, detail of part of one embodiment of selectionlogic 62 is shown. In order to support both scalar and vectormultiplication, selection logic 62 may comprise a plurality ofmultiplexers 310A-B, 312A-B, 314A-B, and 316A-B. These multiplexersoperate to select particular bits from partial product generator 60according to the status of vector₋₋ in signal 120. Each partial producthas its own set of selection multiplexers (excluding constants +0 and -0which are simply fed through as is; see 320A and 320B). For example,multiplexer 310A selects bits [9-0] from the partial product -2M andoutputs them to the rest of selection logic 62 and adder 64 if vector₋₋in is asserted. This may ensure that both effective sign bits 92A and172A are conveyed to adder 64. Two effective sign bits are neededbecause two separate multiplications are being performed. Conversely, ifvector₋₋ in is unasserted (indicating a scalar multiplication), extraeffective sign bit 172A is not needed, thus multiplexer 310A selectsbits [9-6, 4-0] and outputs them as bits [0-8]. The extra effective signbit 172A is removed, and a constant zero is padded to the output tocreate bit [9]. As indicated in the figure, bit [S1] may be passedthrough as it is needed in both cases (scalar and component-wise vectormultiplication). Multiplexer 310B selects bit [S2] if vector₋₋ in signal10 is asserted, thereby providing two constants 88A and 170A. Ifvector₋₋ in signal 120 is not asserted and scalar multiplication isbeing performed, bit [S2] is not needed (and may cause an incorrectresult if it is passed through to adder 64). Thus, multiplexer 310B isconfigured to select and convey a constant zero in place of actual S2bit 170A if scalar multiplication is performed. Multiplexers 312A-B,314A-B, and 316A-B operate in a similar fashion. Each multiplexer may beconfigured to select the required bits from partial product generator 60without passing extra bits unless they are needed.

Turning now to FIGS. 12A-B, more details of one embodiment of selectionlogic 62 and adder 64 are shown. In this embodiment, selection logic 62comprises a plurality of multiplexers 94A-94F as in the previousembodiments. Note that multiplexers 312A-B, 314A-B, and 316A-B are notshown, but are instead included within partial product generator 60.Selection logic 62 further comprises multiplexers 152-156, which operateto select two portions of partial products: (1) a portion of the partialproduct corresponding to the higher order bits of vector operand B1, and(2) a portion of the partial product corresponding to the lower orderbits of vector operand B2. Multiplexer 156 then selects this"combination" partial product when vector₋₋ in signal 120 is asserted.Advantageously, this configuration may remedy the problem of summationcorruption when a bit field encompassing bits from more than one vectoroperand is used to select a partial product. This problem is describedin greater detail below (see FIGS. 13 and 14).

In this embodiment, adder 64 comprises three pluralities of multiplexers160A-160D, 162A-162E, and 164C-164E. Multiplexers 160A-160D arecontrolled by vector₋₋ in signal 120 and operate to "zero-out" portionsof the partial products to prevent corruption of the vector componentswithin final product 76 during the summation within adder 64.Multiplexers 164C-E are also controlled by vector₋₋ in signal 120 andoperate to select either extra constant bits 140C-140E (in the event ofa vector multiplication) or a zero constant (in the event of a scalarmultiplication) for addition into the more significant product.Multiplexers 162A-162D are controlled by sign₋₋ in input 78 and areconfigured to select either the effective sign bit of the moresignificant portion of the selected partial product (in the event of asigned vector multiplication) or the actual sign (in the event of anunsigned vector multiplication). Multiplexers 164C-164E are alsocontrolled by vector₋₋ in signal 102 and perform the same function asmultiplexers 310B, 312B, 314B, and 316B, i.e., they select a constantzero instead of extra constant bit S2 if scalar multiplication isperformed. Note that other configurations of logic for zeroing out andpartial product selection are possible and contemplated. Further notethat multiplexers 160A-160D, 162A-162E, and 164C-164E may be configuredas part of adder 64, selection logic unit 62, or as a separate part ofmultiplier 50.

In addition to the features disclosed above, adder 64 may furthercomprise a plurality of multiplexers (not shown) to prevent carriesacross the boundaries of vector operands within final product 76 whensumming the selected partial products. This boundary is represented by adashed line 178 in the figure. Other embodiments of multiplier 50 mayutilize different configurations of multiplexers. For example,multiplexers 160A-160C may be configured to select either additionalsign-extension bits or the most significant bits of the selected partialproducts. In addition, multiplexers 160A-160C may be configured to padeach selected partial product with prefix bits until the mostsignificant bit of each selected product corresponds to the mostsignificant bit of final product 76 (as indicated by dashed bitpositions 170A-170B). The prefix bits may comprise a constant, signextension bits, or a combination thereof.

Note that FIGS. 11A-B and 12 together illustrate the exemplarycomponent-wise multiplication of two vector operands, i.e., multiplieroperand 74 having a value of (3,12), i.e., B2=3 and B1=12, andmultiplicand operand 72 having a value of (6,7), i.e., A2=6, and A1=7,resulting in final product 76 having a value of (18,84). Further notethat while the figures and exemplary embodiments have illustrated amultiplier configured to perform component-wise vector multiplication onvector operands having up to two vector components, other configurationsare possible and contemplated, e.g. vectors having four or six vectorcomponents may be multiplied component-wise in parallel. Furthermore, anumber of multipliers configured similarly to multiplier 50 may be usedin parallel to achieve even higher performance. The widths of multiplieroperand 74 and multiplicand operand 72 may also be varied, e.g., 32-bitsor 64-bits, as may the widths of their vector components.

In addition, other embodiments of multiplier 50 may be configured toreturn only a portion of final product 76 per clock cycle. For example,the most significant vector component of final product 76 may bereturned during a first clock cycle. Other vector components may bereturned during subsequent clock cycles in order of their significance.

Turning now to FIG. 13, another embodiment of multiplier 50 is shown. Inthis embodiment, multiplier 50 further comprises multiplexer 138. Whenvector₋₋ in signal 120 is asserted, component-wise vector multiplicationis performed. If the summing of partial products generates one or morecarry bits 140, the upper vector component in final product 144 may becorrupted if carry bits 140 are allowed to propagate across boundary176. To prevent this, multiplier 50 may comprise one or more carrymultiplexers 138 to prevent carry bits from propagating to higher ordervector components within final product 76. When multiplier 50 isperforming scalar multiplication, multiplexers 138 may be configured topropagate carry bits normally. As shown in the figure, in thisembodiment of multiplier 50 the partial products in quadrant 130 arezeroed out such that they will not affect the value of final product144.

Turning now to FIG. 14, another embodiment of multiplier 50 is shown. Inthis embodiment, the partial products in quadrant 130 are not zeroedout. Instead, the selected partial products in quadrant 132 are allowedto sign extend across quadrant 130. In some instances, e.g., when vectorcomponents A1 and B1 have opposite signs, final product 76 will have alower order vector component 142 that will be negative and may result ina sign extensions across quadrant 130. This sign extension may affectthe value of the more significant vector component 144 within finalproduct 76. Multiplexer 146 is configured to insert a constant to besummed with the selected partial products to form final product vectorcomponent 144. The constant (e.g., a binary value of one) is calculatedto compensate for a negative sign extension across final product 144.For example, a negative sign extension may be equivalent to "11111111,"thus adding a constant of one (i.e., "00000001") will negate the effectof the sign extension on result vector component 144. As this signextension occurs only when vector components A1 and B1 have differentsigns, an XOR-gate 148 may be used in conjunction with vector₋₋ in input120 to control multiplexer 146 so that the constant is only added whenfinal product 142 will be negative and a component-wise vectormultiplication is being performed. As illustrated, XOR-gate 148 mayreceive the sign bits (i.e., the most significant bits) of vectorcomponents A1 and B1 as inputs.

Vector Dot Product

Multiplier 50 may also be configured to calculate the "vector dotproduct" or inner product of two vectors. The following exampleillustrates the calculation of a vector dot product. Assuming vector Aequals (x1, x2, x3), and vector B equals (y1, y2, y3), then the vectordot product A•B equals x1y1+x2y2+x3y3. As this example illustrates,calculation of the dot product entails performing a component-wisevector multiplication and then summing the vector component products.

Turning now to FIG. 15, one embodiment of multiplier 50 configured tocalculate the vector dot product is shown. As shown in the figure,partial products 190 are summed within adder 64 to form vector componentproducts 192A-N. Each vector component product 192A-N corresponds to onevector pair within multiplicand operand 72 and multiplier operand 74 aspreviously disclosed. Vector component products 192A-N are then summedusing a plurality of carry-propagate adders 194A-N to form final result196, which may then be output for use by other parts of microprocessor10.

Turning now to FIG. 16, another embodiment of multiplier 50 configuredto calculate the vector dot product is shown. In this embodiment,however, partial products 190 summed by adder 64 are kept in redundantform, i.e., each vector component product 192A-F is represented by morethan one value. For example, each vector component product 192A-F may berepresented by two values, a sum value 198A-F and a carry value 200A-F.A set of carry-save adders (not shown) may be used within adder 64 tosum partial products 192 in redundant form. Advantageously, carry-saveadders may significantly reduce the amount of time and die spacerequired to sum partial products 192. At the single-bit level, acarry-save adder will take three bits of the same significance andproduce a sum value (having the same significance) and a carry value(having a significance one bit higher than the sum value). In contrast,the term "carry-propagate adder" denotes an adder that is not acarry-save adder. In one embodiment, a carry-save adder may beimplemented as a number of independent full adders.

Once vector component products 192A-192F have been formed, they may besummed together using a second set of carry-save adders 202A-J. When thenumber of values remaining to be summed is reduced to two, acarry-propagate adder 204 may be used to perform the final summation.Note, however, that this configuration may require further modificationif multiplier 50 is configured to propagate sign extension and carrybits as illustrated in FIG. 14. The embodiment of multiplier 50illustrated in FIG. 14 relies upon carries from less significantproducts propagating into the more significant ones. In this case,summing partial products 190 and products 192A-F using carry-save addersmay cause final result 196 to be less than the correct result by oneunit-in-the-last-place (ULP) for each product below the most significantproduct. This is because carries from lower products are notincorporated into upper products during carry-save adds.

To ensure that final result 196 is correct when multiplier 50 isconfigured in a manner similar to the embodiment of FIG. 14,carry-propagate adder 204 may be configured to accept summands having awidth equal to the cumulative width of all products 192A-F. Assuming thelength of each operand (multiplier and multiplicand) is n bits wide andcomprises p vector components, each product 192A-F will have a width of2n/p. Thus to accommodate all products 192A-192F, adder 204 may be 2nbits wide or wider. The redundant forms of each product 192-192F (e.g.,sum values 198A-F and carry values 200A-F) are conveyed as inputs toadder 204 (excluding the most significant product 192F). In place of themost significant product 192F, the final two summands remaining from thecarry-save summation of products 192A-192F are input to adder 204 as themost significant inputs. While adder 204 will output a 2n-bit wideresult, only the most significant 2n/p bits comprise the final result196. This configuration advantageously allows adder 204 to propagatecarry bits from lower order products to higher order products, therebyensuring a proper result while still retaining the advantages associatedwith carry-save addition. Furthermore, the cost in die space of having a2n-bit wide carry-propagate adder such as adder 204 may be reduced ifother functions to performed by multiplier 50 also require a widecarry-propagate adder.

As with previous embodiments, this embodiment of multiplier 50 may beconfigured to accept operands having varying widths (n), and varyingnumbers of vector components (p). For example, multiplier 50 may beconfigured to calculate the dot product of two vector operands, each64-bits wide and each having four vector components.

Rounded Products

As previously noted, some embodiments of multiplier 50 may be configuredto conserve hardware resources (e.g., signal lines and registers) byreturning only a portion of the final product (or products, in the caseof component-wise vector multiplication) per clock cycle. For example,the higher order bits of the final product may be returned first, andthen the lower order bits may be returned in subsequent clock cycles.However, in some embodiments it may be advantageous to return the higherorder bits rounded to the nearest unit in the last place ("ULP").

Turning now to FIG. 17, a diagram of another embodiment of multiplier 50is shown. This embodiment is configured to round the higher order bitsof each vector component product to the nearest ULP. As in the previousembodiment (illustrated in FIG. 16), partial products 190 are reduced inredundant form (e.g., a sum value and a carry value for each pairs ofvector components) by adder 64. However, in this embodiment a pluralityof adders 210A-210F are used to add a rounding constant 214 to eachvector component product. Rounding constant 214 may comprise a singleasserted bit (i.e., a "one-hot") added to the bit position below theleast significant bit position in the portion of the vector component tobe rounded. For example, assuming a vector component product has a widthof 8 bits, and the four most significant bits (MSBs) are to be rounded,then a constant one would be added to the fourth bit (as illustrated inTable 2). By adding a constant one in the appropriate bit position, theupper portion of the vector component product may be rounded efficientlyand without large amounts of additional logic.

                  TABLE 2                                                         ______________________________________                                        Bit Number ->                                                                             7 (MSB)  6     5   4   3   2   1   0 (LSB)                        ______________________________________                                        Vector Component                                                                          0        1     1   0   1   0   1   1                              Product                                                                       Rounding Constant                                                                         0        0     0   0   1   0   0   0                              Rounded MSBs                                                                              0        1     1   1                                              Output                                                                        ______________________________________                                    

As shown in FIG. 17, each adder 210A-210F is configured to receive theredundant form of a single vector component product. For example, adder210A is configured to receive sum value 198A and carry value 200A andcombine them with rounding constant 214. Adder 210A combines these threevalues and generates a redundant form output comprising a new sum valueand a new carry value. Advantageously, adders 210A-210F may beconfigured as independent carry-save adders, thereby preventingcarry-bits caused by rounding constant 214 from propagating to moresignificant vector component products. The outputs of each adder210A-210F are coupled to the inputs of one of a plurality ofcarry-propagate adders 212A-212F. Each carry-propagate adder 212A-212Fis configured to sum the outputs of adders 210A-210F and therebygenerate a non-redundant form of each vector component product. Therounded MSBs of each vector product may be output first, while theremaining least significant bits ("LSBs") may be output during asubsequent clock cycle. Adders 212A-212F may be configured independentlyto avoid the possibility of an unwanted carry-bit propagating acrossvector product boundaries.

In another embodiment, additional adders (not shown) may be configuredto generate the LSBs (which are unrounded) separately from the MSBs.Advantageously, this may prevent the rounding process from altering thevalue of the LSBs. For example, adder 212A may be configured to generatethe rounded MSBs by summing the sum and carry values generated by adder210A, while an additional adder may be configured to sum the lower bitsof sum value 198A and carry value 200A to generate the LSBs.

In the previously described embodiments, each adder 210A-210F and212A-212F is configured to perform addition without propagating carrybits from one vector component product to another. While this may bedesirable in many configurations, the non-propagation of carry bits maydisrupt some configurations of adder 50. For example, the embodimentillustrated in FIG. 14 relies upon the propagation of sign extensionbits across vector component product boundaries. If carry bits are notallowed to propagate during the final addition stages which convert theredundant-from vector component products to non-redundant-form, thehigher order products may be incorrect.

Turning now to FIG. 18, an embodiment of multiplier 50 which rounds thehigher order bits of each vector component product, yet still allowscarry bits to propagate across consecutive vector component productboundaries, is shown. In this embodiment, rounding constant 214 is onceagain added to the redundant form sum values 198A-198F and carry values200A-200F of each vector component product by carry-save adders210A-210F. In order to allow carries from partial products 190 topropagate without allowing carries from rounding constant 214 topropagate, separate carry-propagate adders 212A-212F are used for eachvector component product. The length of each adder 212A-212F may equalthe number of bits in the vector component product itself plus all ofthe bits corresponding to less significant vector component products.For example, assuming each vector component product is eight bits wide,adder 212B may be 16 bits wide and may add redundant vector componentvalues 198A-198C and 200A-200C. Advantageously, undesired carry-out bitsfrom each vector component product will not affect higher order vectorcomponent products in this configuration. Furthermore, the carry bitsthat may be required for correct operation of the embodiment ofmultiplier 50 illustrated in FIG. 14 still propagate to form the correctresult despite possible sign-extensions.

Note that other configurations of multiplier 50 are possible. Forexample, rounding constant 214 may be incorporated within the logic ofadder 64, thereby potentially eliminating the need for an extra level ofadders. Furthermore, multiplier 50 may be configured to round and returnthe upper portions of scalar products and vector dot products inaddition to vector component products. The types of adders used may alsobe changed according to the implementation, e.g., carry-propagate addersmay be used through out in conjunction with multiplexers configured toprevent carry bits from propagating across vector component productboundaries. In addition, various control signals, e.g., a round₋₋ insignal, may be used to indicate whether rounding is to be performed.

Fast Rounding and Normalization

Another possible area for improving the speed of multiplication relatesto rounding and normalization. When performing floating pointmultiplication, the multiplier and multiplicand operands (i.e., thesignificands of two floating point numbers) are received in normalizedform. A binary number is said to be normalized when the most significantasserted bit is directly to the left of the binary radix point. Forexample, 1.010011₂ is normalized, while 10.10011₂ and 0.01010011₂ arenot. In order to normalize a binary number, the number is shifted eitherright or left until the most significant asserted bit is directly to theleft of the binary radix point. The number's exponent is then increasedor decreased an amount equal to the number of positions that the numberwas shifted.

When multiplier 50 performs floating point multiplication, it receivestwo normalized significands. In some embodiments, multiplier 64 may beconfigured to output the results in normalized form. For example,multiplier 50 may receive two 32-bit normalized significands as operandsand be configured to output one 32-bit result in normalized form. Aftermultiplier 50 generates and selects the partial products, they aresummed by adder 64 to create the final result. As the final result maybe in redundant form, it may be passed through a carry-propagate adderas previously described. Once in non-redundant form, the result isrounded and normalized before being output. Different methods ofrounding are possible. For example, IEEE Standard 754 defines fourdifferent rounding methods: round to nearest (even), round to positiveinfinity, round to minus infinity, and round to zero. The round tonearest method is particularly useful because it ensures that the errorin the final product is at most one-half ULP (unit in the last place).

Turning now to FIG. 19, another embodiment of multiplier 50 is shown.This embodiment comprises two "paths" which are configured to performIEEE rounding and normalization by calculating two results in parallel,i.e., one result assuming there is an overflow and one result assume nooverflow. This embodiment comprises a pair of carry-save adders 276A-B,a pair of carry-propagate adders 278A-B, a pair of sticky bit logicunits 286A-B, and a pair of LSB fix-up logic units 288A-B. The"no-overflow path" comprises carry-save adder 276A, carry-propagateadder 278A, sticky bit logic unit 286A, and LSB fix-up logic unit 288A,while the "overflow path" comprises carry-save adder 276B,carry-propagate adder 278B, sticky bit logic unit 286B, and LSB fix-uplogic unit 288B. Both carry-save adders 276A and 276B are configured toreceive sum value 274A and carry value 274B from partial product arrayadder 64. Each carry-save adder 276A and 276B is also configured toreceive a rounding constant 268 from multiplexer 266.

Multiplexer 266 is configured to select rounding constant 268 from oneof four rounding constants. The first rounding constant is a hard-wiredconstant one and is selected when rounding mode input 270 indicates thatround to nearest (even) is the selected rounding mode. The constant isadded to the guard bit position by both carry save adders 276A and 276B.The second rounding constant is a hard-wired zero and is selected whenrounding mode input 270 indicates that round to zero (truncate) is theselected rounding mode. The third rounding constant is the sign of thefinal product of the multiplication being performed. This sign may beobtained by exclusively ORing the sign bit 260A of multiplicand operand72 and the sign bit 260B of multiplier operand 74 within XOR gate 262.The resulting sign bit is added to the guard bit position, and each bitposition less significant than the guard bit position, by carry-saveadders 276A and 276B. The fourth rounding constant is the inversion ofthe third rounding constant. It may obtained by inverting the roundingconstant obtained from XOR gate 262 with inverter 264. The resultinginverted sign bit is added to the guard bit position and each bitposition less significant than the guard bit position by carry-saveadders 276A and 276B.

Carry-save adders 276A and 276B are configured to receive and add sumvalue 274A, carry value 274B, and the selected rounding constant frommultiplexer 266. Carry-save adders 276A and 276B convey their results inredundant form to carry-propagate adders 278A and 278B, respectively.Carry-propagate adders 278A and 278B reduce the results to non-redundantform 282A and 282B and convey them to LSB fix-up logic units 288A and288B, respectively.

In parallel with the addition performed by adders 276A-B and 278A-B,sticky bit logic units 280A-B calculate sticky bits 286A-B. Sticky bitlogic units 280A-B each receive sum value 274A and carry value 274B asinputs. The calculation of sticky bits and the operation of sticky bitlogic units 280A-B are described in greater detail below.

LSB fix-up logic units 288A and 288B are coupled to carry-propagateadders 278A-B and sticky bit logic units 280A-B. Fix-up logic units288A-B are configured to conditionally invert the least significant bitof the non-redundant results received from adders 278A-B. In oneembodiment, fix-up logic units 288A-B are configured to perform theinversion or "fix-up" when the "round to nearest" mode is beingperformed and the following equation is true: (inverse ofL)·(G)·(inverse of S)=1, wherein L and G are the least significant bits(LSBs) and guard bits, respectively, of the sum of sum value 274A andcarry value 274B, and wherein S is the corresponding sticky bit (either286A or 286B). Note that L and G may be calculated within fix-up units288A-B using sum value 274A and carry value 274. The calculation of Land G may be performed in parallel with the additions performed byadders 276A-B and 278A-B and need not include a rounding constant. L andG may be calculated within fix-up units 288A-B, or by using an extracomponent within multiplier 50 (e.g., a third pair ofcarry-save/carry-propagate adders). The fix-up may advantageouslycompensate for cases in which adders 276A-B have added a constant when aconstant was not actually needed (e.g., result+1 is generated whenresult+0 is needed).

Next, the desired number of upper bits from the outputs of LSB fix-uplogic units 288A and 288B may be conveyed to multiplexer 290, whichselects one of the two values (overflow or no overflow) as output 292.Multiplexer 290 may be controlled by MSB 284 from the output of fix-uplogic unit 288A. By looking at the most significant bit, a determinationof whether an overflow occurred can be made. If an overflow occurred,the upper bits from the output of LSB fix-up logic unit 288A areselected. If an overflow did not occur, the upper bits from the outputof LSB fix-up logic unit 288B are selected. Note that other controlconfigurations are also possible, e.g., MSB 284 may be the mostsignificant bit of the output from fix-up logic unit 288B. Furthermore,in some embodiments of multiplier 50 only one fix-up logic unit may beneeded. For example, the single fix-up logic unit may be coupled to theoutput of multiplexer 290 and perform the fix-up before final result 292is output.

In one embodiment, exponent control logic unit 254 is also controlled bythe same signal that controls multiplexer 290. If an overflow occurs,exponent control logic unit 254 is configured to increment thecorresponding exponent. This completes the normalization of the output.

Advantageously, the embodiment of multiplier 50 depicted in the figuremay be able to round and normalize the final result in less time becausenormalization is performed in parallel. Furthermore, the fix-up isperformed while multiplexer 290 is selecting a result (overflow or nooverflow). This may further reduce the cycle time of this embodiment ofmultiplier 50.

Turning now to FIG. 20, a diagram illustrating the operation of oneembodiment of carry-save adders 276A and 276B is shown. The exampleassumes eight bit sum and carry values 274A-B are being rounded to fourbit values and that round to nearest (even) is being performed. Adders276A-B each receive sum value 274A, carry value 274B, and roundingconstant 268 as inputs. In the example shown, adder 276A is configuredto add a constant one to the guard bit position of sum value 274A andconstant value 274B assuming there will not be an overflow. The guardbit position is the bit position that is one bit less significant thanthe least significant bit of the portion to be output. An overflowoccurs when the summation of sum value 274A, carry value 274B, and anyadded rounding constants, creates a carry out from the bit positiondirectly to the left of the binary radix point. An overflow may requirethe result to be shifted to the right (and the corresponding exponent tobe incremented) in order to produce a normalized output.

As the figure illustrates, adder 276A adds a constant one to the guardbit position of sum value 274A and carry value 274B assuming there willbe no overflow. In contrast, adder 276B adds rounding constant 268 tothe guard bit position of sum value 274A and carry value 274B assumingthere is an overflow. Thus, adder 286B adds the constant one in adifferent bit position than adder 276A. For this reason, adders 276A and276B each generate a different result. The results from adder 276A areconveyed to carry propagate adder 278A, which is configured to reducethem to non-redundant form. Similarly, the results from adder 276B areconveyed to carry propagate adder 278B, which operates in manner similarto adder 278A.

Turning now to FIG. 21, more detail of one embodiment of sticky bitlogic unit 280A is shown. As the figure illustrates, sticky bit logic280A receives the lower four bits of the sum and carry values (350 and352, respectively) generated by adder 276A. A constant 354 (e.g., 1111)is added to the sum and carry bits within carry save adder 340A, therebygenerating two different 4-bit outputs which are routed to exclusive NORgate 342A. The output from exclusive NOR gate 342A is routed to 4-inputOR gate 344A, which outputs sticky bit 286A. Sticky bit logic 280B isconfigured similarly to sticky bit logic 280A, but it may be configuredto receive one extra bit, e.g., five bits as opposed to four bits, dueto the assumed overflow.

Turning now to FIG. 22, a numerical example of the operation of theembodiment of multiplier 50 from FIG. 20 is shown. This example assumesan eight bit output from adder 64 is being rounded to a four bit result.The figure shows each of the four IEEE rounding modes being performed byboth carry-save adders 276A and 276B. The selected rounding constant 268corresponds to the rounding mode. The selected rounding constant 268 isadded to sum value 274A and carry value 274B by carry save adders 276Aand 276B. As the figure illustrates, the starting bit position to whichthe constant is added varies from adder 276A to adder 276B. Aspreviously noted, this is because adder 276A adds the constant to theguard bit position assuming there is no overflow, while adder 276Bassumes there is an overflow. In parallel, sticky bit logic units 280Aand 280B each calculate their own version of the sticky bit (286A and286B, respectively), also reflecting whether or not an overflow ispresumed to occur.

Next, LSB fix-up logic units 288A and 288B fix-up (invert) the LSB ofoutput 282A, if necessary. As the figure illustrates, the fix-up is onlyperformed when round to nearest (even) is the selected rounding mode andthe formula (inverse of LSB)·(Guard bit)·(inverse of Sticky Bit)=1 istrue. Note that in this embodiment the LSB and Guard bit are taken fromthe sum of sum value 274A and carry value 274B without selected roundingconstant 268. After the fix-up, the upper four bits are output tomultiplexer 290. In one embodiment, LSB fix-up logic 288A and 288B mayeach comprise a single inverter configured to invert the leastsignificant bit of results 282A and 282B, respectively.

Other configurations of multiplier 50 are possible and contemplated.Turning now to FIG. 23, another embodiment of multiplier 50 configuredto perform rounding and normalization is shown. In this embodiment, the"fix-up" or inversion of the LSB is performed by a single LSB fix-uplogic unit 288 after multiplexer 290 performs the overflow/no overflowselection. A second multiplexer 290B is included to select which stickybit 286A or 286B will be used by LSB fix-up logic unit 288 indetermining whether to perform the inversion. Note the rounding andnormalization hardware disclosed herein may be configured to round andnormalize redundant results from other functional units also, e.g.,adders.

Fast Newton-Raphson Iteration to Calculate the Reciprocal (1/B)

As microprocessor 10 already contains a highly optimized multiplier 50,it would be advantageous to perform other calculations on multiplier 50as well, e.g., division. This may be accomplished by recasting divisionoperations into reciprocal operations followed by multiplicationoperations. For example, the operation "A divided by B" (A/B) may berecast into "A multiplied by the reciprocal of B" (A×B⁻¹). Forming thereciprocal of B may also be recast into a series of multiplicationoperations by using a version of the Newton-Raphson iteration. TheNewton-Raphson iteration uses the equation X₁ =X₀ ×(2-X₀ ×B) tocalculate the reciprocal of B. The initial estimate, X₀, may bedetermined in a number of different ways. For example, X₀ may be readfrom a ROM table using B as the index, wherein X₀ approximates 1/B. Inanother embodiment, X₀ may be calculated directly from B or from one ormore ROM tables configured to output seed values. The seed values may bemanipulated, e.g., using arithmetic and combinational logic, todetermine X₀. Once X₀ is known, the first iteration may be performed.Thereafter, the results from each iteration are used in place of X₀ insubsequent iterations. This forces X_(n+1) to converge on 1/B in aquadratic fashion.

Turning now to FIG. 24, a flowchart depicting one embodiment of a methodto calculate the reciprocal using multiplier 50 is shown. As previouslynoted, X₀ is calculated first (step 700). Once X₀ is determined, it ismultiplied by B (step 702). The results are then routed down twoparallel paths 706 and 708, one that assumes an overflow took place inthe multiplication (path 706), and another that assumes no overflowoccurred (path 708). Because X₀ is close to 1/B, the product of X₀ and Bwill be close to one, i.e., either slightly over one or slightly underone. As a result, an overflow will only occur during the multiplicationif X₀ is slightly greater than one (i.e., of the form 10.000 . . . withan exponent equal to 2⁻¹). If there is no overflow, the result will beslightly less than one (i.e., in the form 01.111 . . . with an effectiveexponent equal to 2⁻¹).

After the multiplication, the term (2-X₀ ×B) is formed within each pathby inverting the (X₀ ×B) results. Since (X₀ ×B) is close to one, (2-X₀×B) may be approximated by the absolute value of the two's complement of(X₀ ×B). To further speed the calculation, the one's complement may beused because it only differs by a one in the least significant digit.The approximations for (2-X₀ ×B) are performed in parallel within eachpath (steps 710 and 712). Specifically, in overflow path 706, the bitsare inverted to get 01.111 . . . (with an effective exponent equaling2⁻¹). In non-overflow path 708, the bits are inverted to get 10.000 . .. (with an effective exponent equaling 2⁻¹ ). Note that the sign bit ofeach intermediate value may also be forced to zero (positive).

Next, either the overflow path result or the non-overflow path result isselected (step 714). This selection can be performed by examining theresult from the path that assumes no overflow occurred. If the mostsignificant bit of this result is a one, then an overflow occurredwithin the non-overflow path, and the result from the overflow pathshould be selected as the proper result. The corresponding sign andexponent bits are also selected along with the result. Note thatdifferent bits may be selected from each path. This is illustrated bythe following example. Assuming the product from the multiplier is 64bits wide, then the bits may be numbered from 0 (the least significantbit) to 63 (the overflow bit), with the binary radix point locatedbetween the most significant bit 62 and the most significant fractionalbit 61. If an overflow has occurred, bits 62 through 0 are selected withthe radix point positioned between bits 62 and 61. If an overflow hasnot occurred, bits 63 though 0 are selected with the radix pointpositioned between bits 63 and 62. Thus bits 10.000 . . . may beselected as 1.0000 . . . (along with a hardwired exponent equaling 2⁰).Advantageously, this configuration may save time by normalizing theinverted bits without requiring a dedicated normalization step. Notethat other configurations and other widths are contemplated.Furthermore, all the bits from the selected path need not be used. Insome embodiments fewer bits may be selected, and in other embodimentsextra bits may be padded with constants to meet a desired length.

After the appropriate bits are selected, the result is routed back tothe multiplier, which multiplies it with X₀ to complete the firstiteration and form X₁ (step 716). If the desired accuracy has beenachieved (step 718), the results are output (step 722). If the desiredaccuracy has not been achieved (step 720), the iteration is repeated toform X₂, wherein X₂ =X₁ ×(2-X₁ ×B). As with the first iteration, theterm (X₁ ×B) is close to one. The results of the multiplication are onceagain passed down paths 706 and 708 in parallel.

Depending upon the accuracy of the initial guess X₀ and the accuracydesired in the final result, the iteration may be performed any numberof times (e.g., one, two, or five times). Using two paths mayadvantageously eliminate the need for normalization because the exponentand sign bits can be hard-wired based upon the known limits of theincoming operands and whether or not an overflow occurs.

Fast Newton-Raphson Iteration to Calculate the Reciprocal Square Root(1/√B)

In another embodiment, multiplier 50 may be configured to calculate thereciprocal square root of an operand B using a modified version of theNewton-Raphson iteration. The equation Y_(n+1) =Y_(n) ×(3-B×Y_(n) ²)/2may be used to calculate the reciprocal square root of B. Once again,the initial estimate, Y₀, may be determined in a number of ways, e.g.,by using initial estimate generators that perform calculations on seedvalues read from ROM tables using B. In this iteration Y₀ approximatelyequals 1/√B. Each subsequent iteration of the equation forces Y_(n+1) toconverges on 1/√B in a quadratic fashion. In one embodiment, both Y₀ andY₀ ² may be produced using the same initial estimate generator that wasused for the reciprocal calculation described above. This may bedesirable because determining Y₀ ² may eliminate the need for amultiplication operation to form Y₀ ² from Y₀. As used herein, aninitial estimate generator refers to any hardware capable of generatingan initial value such as X₀ or Y₀, e.g., one or more ROM tablesconfigured to output seed values that may be used to calculate theinitial value using arithmetic and combinational logic.

Turning now to FIG. 25, a flowchart depicting one embodiment of a methodto calculate the reciprocal square root using multiplier 50 is shown. Aspreviously noted, Y₀ ² and Y₀ are determined first (step 730). Once Y₀ ²is determined, it is multiplied by B to form the term (B×Y₀ ²) (step732). The results are then routed down two parallel paths 734 and 736,one that assumes an overflow took place in the multiplication (path736), and another that assumes no overflow occurred (path 734). BecauseY₀ ² is close to 1/B, the product of Y₀ ² and B will be close to one,i.e., either slightly over or slightly under one. As a result, anoverflow will only occur during the multiplication if the result (B×Y₀²) is slightly greater than one (i.e., of the form 10.000 . . . with aneffective exponent equal to 2⁻¹). If there is no overflow, the resultwill be slightly less than one (i.e., in the form 01.111 . . . with aneffective exponent equal to 2⁻¹).

After the multiplication, the overflow path 736 forms the one'scomplement by inverting the result (B×Y₀ ²) (step 740). The resultingvalue has the form 01.111 . . . with an effective exponent of 2⁻¹ andapproximates (2-B×Y₀ ²). To form the term (3-B×Y₀ ²), a one iseffectively added to the result to form 1.111 . . . with an exponent of2⁰ (step 744). This value must then be right shifted one bit to reflectthe division by two in the term (3-B×Y₀ ²)/2 (step 748). This results ina value having the form 1.111 . . . with an exponent of 2⁻¹ (step 748).

The non-overflow path 734 also forms the one's complement by invertingthe result (B×Y₀ ²) (step 738). The resulting value, however, has theform 10.000 . . . with an effective exponent of 2⁻¹. This form isnormalized to 1.000 . . . with an exponent of 2⁰, which approximates(2-B×Y₀ ²). To approximate the term (3-B×Y₀ ²), a one is effectivelyadded to the result to form 10.000 . . . (step 742). This value mustthen be shifted right one bit to reflect the division by two in the term(3-B×Y₀ ²)/2 (step 746). The result has the form 1.000 . . . . In thispath, the result's exponent is forced to 2⁰ (step 746).

Next, either the overflow path result or the non-overflow path result isselected (step 750). This selection can be performed as previouslydisclosed, i.e., based upon the value of the most significant bit of theresult from each path. Different bits may be selected from each path toeliminate the need for normalization.

The selected result is then routed back to the multiplier, whichmultiplies it with Y₀ (determined during step 730) to complete the firstiteration and form Y₁ (step 752). If the desired accuracy has beenachieved (step 754), the results are output (step 756). If the desiredaccuracy has not been achieved, the iteration is repeated to form Y₂,wherein Y₂ =Y₁ ×(3-B×Y₁ ²)/2 (step 758). However, unlike the firstiteration, subsequent iterations may require an additionalmultiplication to form the term Y_(n) ² (step 760). As with the firstiteration, the term (B×Y₁ ²) is close to one. Once this term has beencalculated, the results are once again passed down the two paths(overflow 736 and non-overflow 734) in parallel.

Depending upon the accuracy of the initial guess Y₀ and the accuracydesired in the final result, the iterative calculation may be performedany number of times (e.g., one, two, or five times). Advantageously,using two paths (overflow and non-overflow) may eliminate the need fornormalization because the exponent and sign bits may be hard coded basedupon the known limits of the incoming operands and whether or not anoverflow occurs.

Note that the steps in the figures are show in a serial fashion forexplanatory purposes only. Some steps may be performed in parallel or ina different order. Further note that the method above may also be usedto determine the square root of an operand. To implement the square rootfunction, an additional multiplication may be performed during eachiteration.

Turning now to FIG. 26, an embodiment of multiplier 50 configured toevaluate constant powers of an operand is shown. This embodiment may beconfigured to evaluate one or more constant powers of an operand such as-1 (reciprocal), -1/2, (reciprocal square root), and 1/2 (square root).In addition to the features of the previous embodiments, this embodimentof multiplier 50 comprises a non-overflow logic unit 770A, an overflowlogic unit 770B, an initial estimate generator (IEG) 774, twomultiplexers 776 and 780, and a control logic 778. Note thatnon-overflow logic unit 770A and overflow logic unit 770B may also bereferred to herein as a "non-overflow path," and "overflow path,"respectively.

Initial estimate generator 774 is coupled to receive multiplier operand74 and communicate initial estimates, e.g., X₀ and Y₀, to multiplexer776. Note as used herein, X₀ =Y₀ ² ≈1/B, and Y₀ ≈1/√B. Multiplexer 776is configured to select the first multiplication operand from eithermultiplicand operand 72 or the initial estimate output by initialestimate generator 774. Similarly, multiplexer 780 is configured toselect the second operand to be multiplied from either multiplieroperand 74 or result 292 from multiplexer 290. Control logic 778receives control signal 772 and controls multiplexers 776 and 780,exponent control logic 254, and logic units 770A-B. Non-overflow logic770A is coupled to receive values from LSB fix-up logic 288A and outputvalues to multiplexer 290. Similarly, overflow logic 770B is coupled toreceive values from LSB fix-up logic 288B and also output values tomultiplexer 290. Logic units 770A-B are controlled by control logic unit778, which indicates which, if any, constant power operation is beingperformed. If a constant power operation is not being performed, logicunits 770A-B may be configured to simply allow values from fix-up logicunits 288A-B to propagate through to multiplexer 290 unchanged.

When a constant power operation is being performed, logic units 770A-Bare configured to form approximations by inverting selected bits fromthe values received from fix-up logic units 770A-B. Logic units 770A-Bare also configured to force (e.g., hard-wire) the exponents associatedwith the values received from fix-up logic units 288A-B to fixed values.These fixed exponents are communicated to exponent control logic 254.Alternatively, exponent control logic 254 may force the exponents tofixed constants when instructed to do so by logic units 770A-B. Logicunits 770A-B may each comprise a plurality of inverters, a wire shifter,and one or more hard-wired constants. A wire shifter is a plurality ofsignal, data, or control lines that are selectively connected and oroffset to provide fixed shifting and routing. The following examplesillustrate the operation of logic units 770A-B and multiplier 50 in moredetail.

Example of a Reciprocal Operation

When a reciprocal operation is performed, multiplier 50 receives theoperand to be inverted (referred to herein as "operand B") as multiplieroperand 74. Initially, multiplexer 780 is configured to select operandB. Initial estimate generator 774 also receives operand B and inresponse outputs an initial estimate or approximation of the reciprocal(referred to as X₀) to multiplexer 776. Multiplexer 776 is configured toselect, based upon control signals from control logic 778, the initialestimate, which is then multiplied by operand B to form the quantity (X₀×B). The quantity (X₀ ×B) propagates through multiplier 50 until itreaches logic units 770A-770B. Non-overflow logic unit 770A receives aversion from fix-up logic 288A that assumes no overflow has occurred.Based upon control signal 772, non-overflow logic unit 770A inverts theversion of (X₀ ×B) it receives to approximate the quantity (2-X₀ ×B).Non-overflow logic unit 770A may be configured to normalize its outputby forcing the corresponding exponent to a constant, e.g., 2₀. Note allreferences herein are to unbiased exponents. For example, an unbiasedexponent 2⁰ may translate to a biased exponent of 2^(7F) (assuming a+7F₁₆ or +127₁₀ bias). Similarly, overflow logic unit 770B receives aversion from fix-up logic 288B that assumes an overflow has occurred andinverts it. Overflow logic unit 770B may also be configured to normalizeits output by forcing the corresponding exponent to a constant, e.g.,2⁻¹. Note that in some embodiments, not all bits from fix-up logic units288A-B may be used or inverted by logic units 770A-B.

Once the overflow and non-overflow approximations for the quantity (2-X₀×B) have been output by logic units 770A-B, multiplexer 290 isconfigured to select one of the approximations based upon the value ofMSB 284 from the output of fix-up logic unit 288A. As previously noted,by looking at the most significant bit a determination of whether anoverflow occurred can be made. If an overflow occurred, theapproximation for the quantity (2-X₀ ×B) from logic unit 770B (theoverflow path) is selected. If an overflow did not occur, theapproximation for the quantity (2-X₀ ×B) from logic unit 770A (thenon-overflow path) is selected. Note that other control configurationsare possible, e.g., MSB 284 may be the most significant bit of theoutput from fix-up logic unit 288B.

Once the appropriate approximation for the quantity (2-X₀ ×B) has beenselected by multiplexer 290, it is routed to multiplexers 776 and 780.Multiplexer 780 is directed by control logic 778 to select theapproximation so that it may be multiplied by initial estimate X₀ toform the quantity X₀ ×(2-X₀ ×B). During this multiplication, however,logic units 770A-B are configured to allow the values from fix-up logicunits 288A-B to pass through unchanged. The result selected bymultiplexer 290 is the approximation of the reciprocal of operand Bafter one Newton-Raphson iteration. As previously noted, the process maybe repeated a number of times to achieve greater accuracy.

Example of a Reciprocal Square Root Operation

When a reciprocal square root operation is performed, multiplier 50operates in much the same fashion as previously described for areciprocal operation. The operand to be raised to the -1/2 power(referred to herein as "operand B") is received as multiplier operand74. Initially, multiplexer 780 is configured to select operand B.Initial estimate generator 774 also receives operand B and in responseoutputs an initial estimate or approximation of the reciprocal (referredto as Y₀ ², which equals X₀) and the reciprocal square root (referred toas Y₀) to multiplexer 776. Multiplexer 776 is configured to select,based upon control signals from control logic 778, the initial estimateY₀ ², which is then multiplied by operand B to form the quantity (Y₀ ²×B). The quantity (Y₀ ² ×B) propagates through multiplier 50 until itreaches logic units 770A-770B. Non-overflow logic unit 770A receives aversion from fix-up logic 288A that assumes no overflow has occurred.Based upon control signal 772, non-overflow logic unit 770A inverts theversion of quantity (Y₀ ² ×B) it receives to approximate the quantity(2-Y₀ ² ×B). Logic unit 770A also pads the most significant bit of thequantity (2-Y₀ ² ×B) with a constant one to approximate the quantity(3-Y₀ ² ×B). Logic unit 770A may then normalize the quantity (3-Y₀ ² ×B)by selectively routing (e.g., wire-shifting) bits to multiplexer 290 ina particular position or offset and by forcing the correspondingexponent to 2⁰.

Overflow logic unit 770B may be similarly configured to invert theversion of quantity (Y₀ ² ×B) it receives to approximate the quantity(2-Y₀ ² ×B). Logic unit 770B also pads the most significant bit of thequantity (2-Y₀ ² ×B) with a constant one to approximate the quantity(3-Y₀ ² ×B). Logic unit 770B may then normalize the quantity (3-Y₀ ² ×B)by selectively routing bits to multiplexer 290 in a particular positionor offset and by forcing the corresponding exponent to 2⁻¹. Note that insome embodiments, not all bits from fix-up logic units 288A-B may beused or inverted by logic units 770A-B.

Once the overflow and non-overflow approximations for the quantity (3-Y₀² ×B) have been output by logic units 770A-B, multiplexer 290 isconfigured to select one of the approximations based upon the value ofMSB 284 from the output of fix-up logic unit 288A. As previously noted,by looking at the most significant bit a determination of whether anoverflow occurred can be made. If an overflow occurred, theapproximation for the quantity (3-Y₀ ² ×B) from logic unit 770B (theoverflow path) is selected. If an overflow did not occur, theapproximation for the quantity (3-Y₀ ² ×B) from logic unit 770A (thenon-overflow path) is selected. Other control configurations are alsopossible.

Once the approximation for the quantity (3-Y₀ ² ×B) has been selected bymultiplexer 290, it is routed to multiplexer 780. Multiplexer 780 isdirected by control logic 778 to select the approximation so that it maybe multiplied by initial estimate Y₀ that was read from initial estimategenerator 744 to form the quantity Y₀ ×(3-Y₀ ² ×B). During thismultiplication, however, logic units 770A-B are configured to allow thevalues from fix-up logic units 288A-B to pass through unchanged. Theresult selected by multiplexer 290 is the approximation of thereciprocal square root of operand B after one Newton-Raphson iteration.As previously noted, the process may be repeated a number of times toachieve greater accuracy. However, in subsequent iterations the resultmust be squared to form Y_(n) ², which is then used in place of theinitial estimate Y₀ ² from initial estimate generator 774.

Note other configurations of multiplier 50 are possible andcontemplated. For example, non-overflow logic 770A and overflow logic770B may instead be configured to receive rounded and normalized value292 from multiplexer 290, in which case a separate multiplexer (notshown) may be needed to select between the values output by non-overflowlogic 770A and overflow logic 770B. In some embodiments of multiplier50, registers may be used to store various intermediate results, e.g.,the inputs to multipiexers 776 and 780, and the results from multiplexer290. The registers may the store the intermediate results for use duringsubsequent clock cycles.

Turning to FIG. 27, details of one exemplary embodiment of non-overflowlogic unit 770A configured to calculate the quantity (3-Y₀ ² ×B) for thereciprocal square root calculation are shown. When the quantity (Y₀ ²×B) 790A is received from LSB fix-up logic 228A, inverters 792A invertselected bits to approximate the quantity (2-Y₀ ² ×B). Note that aninverter may not be required for all bits, e.g., the most and leastsignificant bits. Constants 794A are then used to replace the mostsignificant bit of the quantity (2-Y₀ ² ×B) 790A to approximate thequantity (3-Y₀ ² ×B), which is output to multiplexer 290. A constant orcontrol signal may be routed to exponent control logic 254 to force thecorresponding exponent to 2⁰.

A numerical example further illustrates the operation of non-overflowlogic unit 770A. First, the value 1.111 . . . ×2⁻¹ is received from LSBfix-up logic 228A as an approximation of the quantity (Y₀ ² ×B) 790A.Next, inverters 792A invert the quantity to generate 10.000 . . . ×2⁻¹as an approximation of the quantity (2-Y₀ ² ×B). Finally, constants 794Aare used to replace the most significant bits. The results are shifted,resulting in the quantity 1.00000 . . . , and the corresponding exponentis forced to 2⁰. Note that the most and least significant bits of thequantity (Y₀ ² ×B) 790A may not be incorporated into the quantity (2-Y₀² ×B).

Overflow logic 770B operates in a similar fashion. However, the mostsignificant bit of quantity 790B is replaced with only a single constant794B, and bits 30 through 0 are incorporated into the quantity (2-Y₀ ²×B). A numerical example further illustrates the operation of overflowlogic unit 770B. First, the value 10.000 . . . ×2⁻¹ is received from LSBfix-up logic 228B as an approximation of the quantity (Y₀ ² ×B) 790B.Next, inverters 792B invert the quantity to generate 01.111 . . . ×2⁻¹as an approximation of the quantity (2-Y₀ ² ×B). Finally, constant 794Bis used to replace the most significant bit. The results are shifted,resulting in the quantity 1.1111 . . . , and the corresponding exponentis forced to 2⁻¹.

Turning now to FIG. 28, another exemplary embodiment of non-overflowlogic 770A and overflow logic 770B is shown. The embodiments shown inthe figure are configured to return the quantity (2-Y₀ ² ×B) for thereciprocal calculation. A numerical example illustrates the operation ofnon-overflow logic 770A and overflow logic 770B. Assuming non-overflowlogic 770A receives a value 1.111 . . . ×2⁻¹ as the quantity (Y₀ ² ×B)790A, then inverters 792A ar used to invert the bits (excluding theleast significant bit) to obtain a value 0.000 . . . . Constant 796A isthen used to pad the most significant bit position. The remaining bitsare all shifted one position, with the result 1.0000 . . . being outputto multiplexer 290. The corresponding exponent is forced to 2⁰ by signal796A.

Overflow path 770B operates in a similar fashion. For example, assumingvalue 790B is 1.000 . . . ×2⁻¹, then inverters 792B generate the value0.111 . . . which is shifted and output to multiplexer 290 as the value1.11 . . . . Note the least significant bit may be padded with aconstant 794B, e.g., zero, while the corresponding exponent is forced to2⁻¹ by signal 796B.

Note the examples and figures referred to herein are exemplary. Otherconfigurations for non-overflow logic 770A and overflow logic 770B andmultiplier 50 are also contemplated. For example, the least significantbit from quantity 790B may be duplicated instead of using constant 794B.Other constant values may also be used, and the widths of quantities770A-B may be reduced before they are routed to multiplexer 290 (e.g.,from 32 bits to 24 bits). Other logic components may be used in place ofinverters 792A-B, and the bit routing structure disclosed above may bereplaced by other logic components, e.g., a shifter. The functionalityprovided by non-overflow logic 770A and overflow logic 770B may beprovided in other components internal or external to multiplier 50. Inaddition, multiplier 50 may be configured to perform both reciprocal andreciprocal square root functions, e.g., by incorporating two versions ofnon-overflow logic 770A and overflow logic 770B, or by incorporatingmultiplexers within non-overflow logic 770A and overflow logic 770B toselect which routing of bits and constants should be applied.

Compression of Intermediate Products

When performing iterative calculations, multiplier 50 calculatesintermediate products which may be stored in registers. During the nextiteration, the intermediate product may be read from the register andused as an operand. Unfortunately, each iteration may introduce roundingerrors that accumulate in the final result. For example, assuming anN-bit significand, the results from each multiplication havesignificands that are 2N bits wide. This result may be rounded to N-bitsor some other width. The greater the number of iterations, the largerthe potential rounding error may be in the final result. For obviousreasons, it is desirable to reduce the magnitude of this rounding error.

One possible method to reduce the rounding error is to calculate extrabits for each intermediate product and then round at lower bitpositions. Each iteration may generate accurate bits in lower (lesssignificant) bit positions than the previous iteration. However, due tothe fixed size of the storage registers within multiplier 50, the extrabits will not fit unless the registers within multiplier 50 are widenedaccordingly. There are several potential drawbacks to using widerregisters, including the additional die space requirements and theadditional architectural state requirements for context switches. Thus,a mechanism for maintaining the accuracy provided by the extra bitswithout using wider registers may be desirable.

One possible method for providing such extra accuracy without increasingthe size of the storage registers is to compress the intermediateresults before they are stored. However, not all compression algorithmsare well suited for use within multiplier 50. One concern, inparticular, is speed. Another concern is the die space required toimplement the compression.

Turning now to FIG. 29A, a flowchart illustrating one possible methodfor fast compression is shown. In the embodiment illustrated, theintermediate product is first calculated to N extra significant bits(step 600), wherein N is a predetermined constant. For example, assumingmultiplier 50 receives 24-bit operands, multiplier 50 may calculateintermediate products to a precision of 28 bits. In this case, N equals4 bits. Once the intermediate product is calculated, the next-to-mostsignificant bit is examined (step 602). The value of the next-to-mostsignificant bit determines the value of a signaling bit. If thenext-to-most significant bit equals one (step 604), then the signalingbit equals one also. If, on the other hand, the next-to-most significantbit equals zero (step 606), then the signaling bit equals zero. Thesignaling bit is used to replace a portion of the intermediate product,thereby compressing the intermediate product (step 608). In oneembodiment, the portion replaced by the signaling bit is N+1 bits wide.While this method assumes that the portion being replaced comprisesentirely one's or zero's, this may be a safe assumption when certaintypes of iterations are being performed. For example, when performingthe Newton-Raphson iterations previously disclosed for calculating thesquare root and inverse square root, the products (2-B×X_(n)) and(3-B×Y_(n) ²) are formed during each iteration. As previously noted,these product are very close to one (e.g., either slightly over one,1.00000000 . . . ×2⁰, or slightly under one, 1.11111111 . . . ×2⁻¹).Accordingly, many of the leading bits (excluding the most significantbit in some cases) of the products are identical, i.e., either all zerosor ones, from one iteration to the next with differences occurring inthe less significant bits. This property allows the method illustratedin FIG. 29A to be used effectively.

The maximum number of bits that may be compressed in a particularimplementation may be determined by examining the number of bits thathave the same values over the entire range of possible operand values.For example, if an embodiment using a 32-bit significand is determinedto have nine bits that have the same value for all possible operandvalues, then the 32-bit results may compressed so that they may bestored in 24-bit registers.

While the present embodiment illustrated in the figure performs thecompression whenever a particular iterative calculation is performed, inother embodiments the compression may be performed conditionally. Forexample, in one embodiment the compression may be performed only if acomparison of the bits to be compressed shows that they all have thesame value. While many different types of hardware may be used toperform this comparison, one possible configuration may utilize multipleinput AND gates and multiple input NAND gates. If the testing logicdetermines that the bits to be compressed do not all have the samevalue, then the operand may stored by truncating the extra leastsignificant bits. While this implementation may lose the benefit ofincreased accuracy in some cases, this may be adequate if the bits to becompressed rarely have different values.

When the compressed intermediate product is needed for the nextiteration, it may be decompressed. Turning now to FIG. 29B, one possiblemethod for decompressing the compressed intermediate product isillustrated. First, the compressed intermediate product is read from thestorage register (step 612). Next, the compressed intermediate productis expanded by padding the next-to-most significant bits with copies ofthe signaling bit (step 614). The number of copies of the signaling bitthat are padded or inserted below the most significant bit in thisembodiment equals N-1. Advantageously, the expanded intermediate productnow has the same width as the original intermediate product. Forexample, if the compressed intermediate product comprises 24 bits, andthe original intermediate product comprises 28 bits, then the signalingbit will be copied 4 times to render an expanded intermediate producthaving 28 bits. Advantageously, using the methods illustrated in FIGS.29A and 29B, no information is lost in the compression and decompressionprocess.

Note that the bits replaced by the signaling bit need not be the mostsignificant bit. They may begin with the next-to-most significant bit.For example, if the most significant bit of the intermediate product isbit 27, the bits replaced by the signal bit may comprise bits 22 through26. Further note that the signaling bit may simply be a particular bitwithin the intermediate product, i.e., an extra calculation to determinethe signal bit is not required. Furthermore, the signal bit need not bethe most significant or least significant bit in the range of bits to becompressed, i.e., the signal bit may be a bit in the middle of the rangeof bits to be compressed.

Turning now to FIG. 30, one embodiment of multiplier 50 configured tocompress intermediate products is shown. As in previous embodiments,this embodiment of multiplier 50 comprises partial product generator 60,selection logic 62, and partial product array adder 64. This embodimentalso comprises demultiplexer 622, 24-bit storage register 638, andmultiplexers 776 and 780. Demultiplexer 622 receives an intermediateproduct 620 from partial product array adder 64. Other embodiments arealso contemplated. For example, demultiplexer 622 may receiveintermediate product 620 from multiplexer 290 (see FIG. 26).Demultiplexer 622 routes intermediate product 620 according to aniterative control signal 644. For example, if iterative control signal644 indicates that an iterative operation is being performed, thenintermediate product 620 is routed to storage register 638. If, on theother hand, iterative control signal 644 indicates that an iterativeoperation is not being performed, then intermediate product 620 may berouted to standard rounding logic (not shown) and then output. Inanother embodiment, intermediate product 620 may be rounded beforereaching demultiplexer 622. In this case, demultiplexer 622 may simplyroute product 620 to an output of multiplier 50 if an iterativecalculation is not being performed.

In the event an iterative operation is being performed, storage register638 is configured to store intermediate product 620 until it is neededfor the next iteration. The signal and data lines coupled to the inputsand outputs of storage register 638 may be referred to herein as a wireshifter because they provide a fixed shifting function. As previouslynoted, storage register 638 may be implemented so that it is smallerthan intermediate product 620. Assuming, for example, that intermediateproduct is 28 bits wide, storage register 638 may be configured to storeonly the 24 least significant bits of intermediate product 620. Assumingthe five next-to-most significant bits of intermediate product 620 allhave the same value for the particular iteration being performed, thenbit 22 may be selected as a signal bit 632 to replace the fournext-to-most significant bits 636. Thus, as the figured illustrates,storage register 638 may be configured to store bits 0-21, signal bit632, and bit 27.

When the next iteration is performed, bits 0-21, signal bit 632, and bit27 are read from storage register 638. To recreate the full 28 bits ofintermediate product 620, signal bit 632 is copied four times torecreate bits 23-26. Advantageously, no information from intermediateproduct 620 is lost in the compression and decompression cycle.

Multiplier 50 may also be configured with optional testing logic 624,which is configured to determine whether the five most significant bitsfrom intermediate product 620 have the same value. In one embodiment,testing logic 624 may comprise five-input AND gate 626, five-input NORgate 628, and two-input OR gate 630. The output from two-input OR gate630 may be used in a number of ways, e.g., to signal an error conditionor to cause register 638 to store the 24 most significant bits withoutcompression.

In some embodiments, testing logic 624 may be omitted. Furthermore,demultiplexer 622 may also be omitted. In such embodiments, product 620may be rounded and then routed to both storage register 638 and theoutputs of multiplexer 50. In the event of an iterative calculation,external logic may be used to ensure that functional units or otherparts of the microprocessor will not use the data output by multiplier50 until the iterative calculation is completed

Turning now to FIG. 31A, a figure illustrating one possible method forcompression is shown. As the figure illustrates, uncompressedintermediate product 656 may comprise 28 bits numbered 0 through 27. Ifthe five most significant bits 652 from intermediate product 656 allhave the same value, they may be compressed into one signal bit 654.This allows uncompressed intermediate product 656 to be represented andstored as compressed intermediate product 658, thereby using only 24bits. When compressed intermediate product 658 is uncompressed, signalbit 654 is copied four times to create the four most significant bits ofuncompressed intermediate product 660.

Turning now to FIG. 31B, a figure illustrating another possible methodfor compressing intermediate product is shown. In this embodiment,intermediate product 676 is characterized by having five equal bits 672directly below most significant bit 680. As the figure illustrates, thefive equal bits 672 may be compressed into one signal bit 674 eventhough they are not the most significant bits in intermediate product676. Compressed product 678 is still able to fit within a 24 bit storageregister. To decompress compressed product 678, four copies of signalbit 674 are inserted below most significant bit 680 within uncompressedproduct 682. Once again, no information from intermediate product 676 islost in the process. As this example illustrates, the contemplatedcompression method may be used regardless of where the bits having equalvalues are located. Advantageously, no information is lost if the bitshaving equal values are located in the same position in each iteration.

Achieving Higher Frequencies of Exactly Rounded Results

When an infinitely precise result is rounded to the nearest machinenumber, the maximum possible error is one-half of a unit in the lastplace (ulp). When performing an iterative calculation such as theNewton-Raphson iterations discussed above for the reciprocal andreciprocal square root, the results converse toward the infinitelyprecise result. However, due to limitations in the number of bits ofprecision that are available, the number of iterations performed, andthe approximations discussed above to improve the speed of eachiteration, some input operands may generate results from multiplier 50that do not equal the infinitely precise result rounded to the nearestmachine number (also referred to as the "exactly rounded result"). Thisholds true even when each iteration is configured to use the "round tonearest" rounding mode.

Thus, it would be desirable to increase the frequency or probabilitythat the calculated result equals the exactly rounded result. One methodto determine whether the calculated result equals the exactly roundedresult is to multiply the calculated result (calculated to at least oneextra bit of accuracy, i.e., N+1 bits) and the original operand B(assuming the reciprocal of B has been calculated). The exactly roundedresult may then be selected from the following three values: the N-bitresult (without the extra bit) plus one in the least significant bit;the N-bit result minus one in the least significant bit; or the N-bitresult plus zero. The exactly rounded result is selected based upon thevalue of the extra computed bit (i.e., bit N+1) and whether the resultof multiplication was greater than one, less than one, or equal to one.

Rather than computing the exactly rounded result with a probability ofone as described above (i.e., performing an extra multiplication step),multiplier 50 may be configured to achieve nearly the same accuracy(i.e., computing the exactly rounded result with a probability close toone) by adding an "adjustment constant" to the result produced from thelast step of the iterative calculation before rounding. Depending uponthe actual implementation of the multiplier (e.g., the number of bits ofprecision, the number of iterations performed, and the accuracy of theinitial approximation) the probability that the calculated result ishigher than the exactly rounded result ("P_(high) ") may be greater thanthe probability that the calculated result is lower than the exactlyrounded result ("P_(low) "). If this is the case, then adding a negativeadjustment constant in the final step of the iteration may increase theprobability that the calculated result will equal the exactly roundedresult ("P_(equal) "). Similarly, if P_(high) is less than P_(low), thenadding a positive adjustment constant may increase P_(equal). Theprobabilities P_(high), P_(low), and P_(equal) may be determined bypassing a large number of differing input operand values through theiterative calculation (as performed by the multiplier) and thencomparing each result with the corresponding exactly rounded result. Acomputer program may be particularly useful in performing thecomparisons. The comparisons may also be performed before rounding,i.e., comparing the infinitely precise results with the results from themultiplier's final iteration before they are rounded.

Turning now to FIG. 32, an embodiment of multiplier 50 configured to adda correction constant is shown. Generally, multiplier 50 may beconfigured similarly to other embodiments disclosed herein that arecapable of performing iterative calculations. However, in thisembodiment control logic 778 is configured to convey adjustment constant800 to partial product array adder 64 during the last step of aniterative calculation. Partial product array adder 64 then sumsadjustment constant 800 with the selected partial products fromselection logic 62. Advantageously, this configuration may not requirean additional set of adders to sum adjustment constant 800 into theresult. Another potential advantage of this configuration is that anyoverflows or denormalizations that occur as a result of adjustmentconstant 800 are addressed by the rounding and normalization processalready built into multiplier 50 (i.e., carry-save adders 276A-B,carry-propagate adders 278A-B, sticky bit logic units 280A-B, LSB fix-uplogic units 288A-B, and logic units 770A-B).

In another embodiment, adjustment constant 800 may instead be summedinto the result by carry-save adders 276A-B and or carry-propagateadders 278A-B. Another embodiment may incorporate an extra adder (notshown) to sum adjustment constant 800 with result 292 from multiplexer290. However, this configuration may require additional logic in theevent the result becomes denormalized or experiences an overflow as aresult of the addition.

Control logic 778 may be configured to convey adjustment constant 800 topartial product array adder 64 during the final multiplication in eachiteration, or just for the final multiplication during the finaliteration. For example, if the iterative calculation involves twomultiplication operations for each iteration, and three iterations arerequired to achieve the desired accuracy, then control logic 778 may beconfigured to convey adjustment constant 800 during the finalmultiplication of each iteration, i.e., three times, or only once duringthe second multiplication of the third and final iteration. In yetanother embodiment, control logic 778 may convey adjustment constant 800to partial product adder 64 during every multiplication in theiteration.

In yet another embodiment, control logic unit 778 may store a number ofdifferent adjustment constants 800, e.g., one for each type ofiteration. In such a configuration, control logic unit 778 may conveythe appropriate adjustment constant that corresponds to the type ofiterative calculation being performed. Control logic unit 778 receivesan indication of which iterative calculation is being perform viacontrol signal 722. For example, when control signal 722 indicates thatthe reciprocal iterative calculation is to be performed, control logic778 may convey a first adjustment constant 800 to partial product arrayadder 64. However, when control signal 722 indicates that the reciprocalsquare root iterative calculation is being performed, control logic 778may convey a second, different adjustment constant to partial productarray adder 64.

In another embodiment, multiplier 50 may be configured to calculatethree versions of each result, i.e., a first result generated withoutadding an adjustment constant, a second result generated by adding anadjustment constant, and a third result generated by subtracting theadjustment constant. Alternatively, the second and third results couldbe calculated by adding different adjustment constants. These resultsmay be calculated in parallel by multiplier 50. Once the result withoutthe adjustment constant is generated, a multiplication may be performedas described above to determine whether the result is correct, too high,or too low. The corresponding result may then be selected.

Exemplary Configuration Using Two Multipliers

Turning now to FIG. 33A, an example of a vector multiplication using twomultipliers 50A and 50B is shown. Multipliers 50A and 50B may beconfigured similarly to multiplier 50 as described in previousembodiments. As shown in the figure, multipliers 50A and 50B areconfigured to operate in parallel to execute a vector multiplication ofa pair of vectors each comprising four 16-bit operands 380A-380D and382A-382D. Note operands 380A-380D may come from a first 64-bit MMXregister, while operands 382A-382D may come from a second 64-bit MMXregister.

Turning now to FIG. 33B, another example of a vector multiplicationusing multipliers 50A and 50B is shown. In this configuration,multipliers 50A and 50B operate in parallel to multiply a pair ofvectors each comprising two 32-bit operands 384A-384B and 386A-386B.Once again, operands 384A-384B may come from a first 64-bit MMXregister, while operands 386A-386B may come from a second 64-bit MMXregister. Further note that while a vector operation is being performed,each individual multiplier 50A and 50B is performing a scalarmultiplication. Other modes of operation are also contemplated, forexample, multiplier 50A may perform a 32-bit scalar multiplicationindependent from multiplier 50B. While multiplier 50A performs themultiplication, multiplier 50B may sit idle or perform an independentmultiplication operation.

Exemplary Computer System Using Multiplier

Turning now to FIG. 34, a block diagram of one embodiment of a computersystem 400 including microprocessor 10 is shown. Microprocessor 10 iscoupled to a variety of system components through a bus bridge 402.Other embodiments are possible and contemplated. In the depicted system,a main memory 404 is coupled to bus bridge 402 through a memory bus 406,and a graphics controller 408 is coupled to bus bridge 402 through anAGP bus 410. Finally, a plurality of PCI devices 412A-412B are coupledto bus bridge 402 through a PCI bus 414. A secondary bus bridge 416 mayfurther be provided to accommodate an electrical interface to one ormore EISA or ISA devices 418 through an EISA/ISA bus 420. Microprocessor10 is coupled to bus bridge 402 through a CPU bus 424.

Bus bridge 402 provides an interface between microprocessor 10, mainmemory 404, graphics controller 408, and devices attached to PCI bus414. When an operation is received from one of the devices connected tobus bridge 402, bus bridge 402 identifies the target of the operation(e.g. a particular device or, in the case of PCI bus 414, that thetarget is on PCI bus 414). Bus bridge 402 routes the operation to thetargeted device. Bus bridge 402 generally translates an operation fromthe protocol used by the source device or bus to the protocol used bythe target device or bus.

In addition to providing an interface to an ISA/EISA bus for PCI bus414, secondary bus bridge 416 may further incorporate additionalfunctionality, as desired. For example, in one embodiment, secondary busbridge 416 includes a master PCI arbiter (not shown) for arbitratingownership of PCI bus 414. An input/output controller (not shown), eitherexternal from or integrated with secondary bus bridge 416, may also beincluded within computer system 400 to provide operational support for akeyboard and mouse 422 and for various serial and parallel ports, asdesired. An external cache unit (not shown) may further be coupled toCPU bus 424 between microprocessor 10 and bus bridge 402 in otherembodiments. Alternatively, the external cache may be coupled to busbridge 402 and cache control logic for the external cache may beintegrated into bus bridge 402.

Main memory 404 is a memory in which application programs are stored andfrom which microprocessor 10 primarily executes. A suitable main memory404 comprises DRAM (Dynamic Random Access Memory), and preferably aplurality of banks of SDRAM (Synchronous DRAM).

PCI devices 412A-412B are illustrative of a variety of peripheraldevices such as, for example, network interface cards, videoaccelerators, audio cards, hard or floppy disk drives or drivecontrollers, SCSI (Small Computer Systems Interface) adapters andtelephony cards. Similarly, ISA device 418 is illustrative of varioustypes of peripheral devices, such as a modem, a sound card, and avariety of data acquisition cards such as GPIB or field bus interfacecards.

Graphics controller 408 is provided to control the rendering of text andimages on a display 426. Graphics controller 408 may embody a typicalgraphics accelerator generally known in the art to renderthree-dimensional data structures which can be effectively shifted intoand from main memory 404. Graphics controller 408 may therefore be amaster of AGP bus 410 in that it can request and receive access to atarget interface within bus bridge 402 to thereby obtain access to mainmemory 404. A dedicated graphics bus accommodates rapid retrieval ofdata from main memory 404. For certain operations, graphics controller408 may further be configured to generate PCI protocol transactions onAGP bus 410. The AGP interface of bus bridge 402 may thus includefunctionality to support both AGP protocol transactions as well as PCIprotocol target and initiator transactions. Display 426 is anyelectronic display upon which an image or text can be presented. Asuitable display 426 includes a cathode ray tube ("CRT"), a liquidcrystal display ("LCD"), etc.

It is noted that, while the AGP, PCI, and ISA or EISA buses have beenused as examples in the above description, any bus architectures may besubstituted as desired. It is further noted that computer system 400 maybe a multiprocessing computer system including additionalmicroprocessors (e.g. microprocessor 10a shown as an optional componentof computer system 400). Microprocessor 10a may be similar tomicroprocessor 10. More particularly, microprocessor 10a may be anidentical copy of microprocessor 10. Microprocessor 10a may share CPUbus 424 with microprocessor 10 (as shown in FIG. 5) or may be connectedto bus bridge 402 via an independent bus.

It is still further noted that the present discussion may refer to theassertion of various signals. As used herein, a signal is "asserted" ifit conveys a value indicative of a particular condition. Conversely, asignal is "deasserted" if it conveys a value indicative of a lack of aparticular condition. A signal may be defined to be asserted when itconveys a logical zero value or, conversely, when it conveys a logicalone value. Additionally, various values have been described as beingdiscarded in the above discussion. A value may be discarded in a numberof manners, but generally involves modifying the value such that it isignored by logic circuitry which receives the value. For example, if thevalue comprises a bit, the logic state of the value may be inverted todiscard the value. If the value is an n-bit value, one of the n-bitencodings may indicate that the value is invalid. Setting the value tothe invalid encoding causes the value to be discarded. Additionally, ann-bit value may include a valid bit indicative, when set, that the n-bitvalue is valid. Resetting the valid bit may comprise discarding thevalue. Other methods of discarding a value may be used as well.

Although the embodiments above have been described in considerabledetail, other versions are possible. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A method for increasing the frequency of exactlyrounded results when performing an iterative calculation in amicroprocessor comprising:receiving an operand; determining an initialestimate of the result of said iterative calculation using said operand;multiplying said initial estimate and said operand to generate anintermediate result; repeating said multiplying a predetermined numberof times, wherein said intermediate result is used in place of saidinitial estimate, wherein the final repetition generates a final result;and adding an adjustment constant to said final result, wherein saidadjustment constant increases the probability that said final resultwill equal the exactly rounded result of said iterative calculation. 2.The method as recited in claim 1, wherein said multiplying furthercomprising adding the adjustment constant to said intermediate result.3. The method as recited in claim 1, wherein said multiplying furthercomprises rounding and normalizing said intermediate result.
 4. Themethod as recited in claim 3, wherein said determining comprises readinga plurality of values from a read-only memory using said operand as anindex and arithmetically manipulating said values to form said initialestimate.
 5. The method as recited in claim 4, wherein said adjustmentconstant is determined by performing said iterative calculation for aplurality of operand values to generate a plurality of final results,and selecting a constant that, when added to each of said plurality offinal results, forces a higher percentage of said plurality of finalresults to equal their corresponding exactly rounded results.
 6. Amethod for increasing the probability of exactly rounded results whenusing a multiplier to evaluate a constant power of an operandcomprising:determining an initial estimate of the operand raised to afirst constant power; performing an iteration comprising:multiplying theoperand and the initial estimate in the multiplier to form a product;adding an adjustment constant to said product; calculating anintermediate approximation by inverting one or more bits from saidproduct; rounding said intermediate approximation; normalizing saidintermediate approximation; and repeating said iteration, wherein saidintermediate approximation is used in place of said initial estimate. 7.The method as recited in claim 6, wherein said multiplying and saidadding is performed by summing a plurality of partial products and theadjustment constant in an adder tree.
 8. The method as recited in claim6, wherein said iteration comprises a Newton-Raphson iteration.
 9. Themethod as recited in claim 6, wherein said adjustment constant varieswith said product.
 10. The method as recited in claim 6, wherein saidadjustment constant comprises a value selected from a plurality ofvalues, wherein said value is selected during each iteration based uponthe type of iteration being performed.
 11. An execution unit configuredto evaluate a constant power of an operand comprising:an initialestimate generator configured to receive an operand and output aninitial estimate of said operand raised to a particular constant power;and a multiplier coupled to receive said operand and said initialestimate, wherein said multiplier is configured to calculate a productof said initial estimate and said operand, wherein said multiplier isfurther configured to conditionally add an adjustment constant to saidproduct to form an adjusted product, wherein said multiplier is furtherconfigured to round and normalize said adjusted product, wherein saidadjustment constant is selected to increase the probability that saidadjusted product equals an exactly rounded product.
 12. The executionunit as recited in claim 11, wherein said multiplier comprises a partialproduct array adder configured to sum said adjustment constant and aplurality of partial products, wherein said plurality of partialproducts are a function of said operand and said initial estimate. 13.The execution unit as recited by claim 11, wherein said multipliercomprises a partial product array adder configured to conditionally sumsaid adjustment constant and a plurality of partial products, whereinsaid plurality of partial products are a function of said operand andsaid initial estimate, wherein said multiplier further comprises acontrol unit coupled to said partial product array adder, wherein saidpartial product array adder is configured to sum said adjustmentconstant only in response to a control signal from said control unit.14. The execution unit as recited in claim 13, wherein said control unitis configured to assert said control signal only on final multiplicationcycles of iterative multiplication operations.
 15. A multipliercomprising:a partial product generator coupled to receive a multiplicandinput and configured to generate one or more partial products based uponsaid multiplicand operand; selection logic coupled to said partialproduct generator and configured to receive a multiplier operand,wherein said selection logic is configured to select a plurality ofpartial products based upon said multiplier operand; and a partialproduct array adder coupled to said selection logic, wherein said adderis configured to receive and sum a number of said partial products andan adjustment constant to form a product, wherein said adjustmentconstant is selected to increase the frequency that said product isexactly rounded.
 16. The multiplier as recited in claim 15, furthercomprising:a first plurality of inverters coupled to receive, invert,and normalize selected bits from said product to form a firstapproximation, wherein said first approximation assumes an overflow hasoccurred in said multiplier; a second plurality of inverters coupled toreceive, invert, and normalize selected bits from said product to form asecond approximation, wherein said second approximation assumes anoverflow has not occurred in said multiplier; and a multiplexerconfigured to select either said first or second approximations.
 17. Themultiplier as recited in claim 16, further comprising a control unitcoupled to said partial product array adder, wherein said control unitis configured to prevent said adjustment constant from being summedunless a final step of an iterative calculation is being performed. 18.The multiplier as recited in claim 17, wherein said control unit isconfigured to select said adjustment constant from a plurality ofvalues, wherein each value corresponds to a particular type of iterativecalculation.
 19. The multiplier as recited in claim 17, wherein saidadjustment constant is predetermined and hard-wired into said controlunit.
 20. A microprocessor comprising:an instruction cache; a load/storeunit coupled to said instruction cache; and an execution unit coupled toreceive and execute instructions from said instruction cache, whereinsaid execution unit is configured to evaluate a constant power of anoperand, wherein said execution unit comprises:an initial estimategenerator configured to receive an operand and output an initialestimate of said operand raised to a particular constant power; and amultiplier coupled to receive said operand and said initial estimate,wherein said multiplier is configured to calculate a product of saidinitial estimate and said operand, wherein said multiplier is furtherconfigured to conditionally add an adjustment constant to said productto form an adjusted product, wherein said multiplier is furtherconfigured to round and normalize said adjusted product, wherein saidadjustment constant is selected to increase the probability that saidadjusted product equals an exactly rounded product.